Phendrana Drifts
Now that you're in Phendrana it's time to find the next upgrade here.
Phendrana Shorelines
There is only one reachable door not covered yet, and to reach it you scale the ledges starting to the right of the Save Station door. But the door is locked for the moment and a scan tells you that the controls are somewhere else.
You don't actually have to go far; the console to unlock it is in the protruding tunnel just beneath. First fire a missile to clear the grate at the entrance, then switch to Morph Ball and go over the snow drift. Once inside scan the console to unlock the door, get back and go through.
Ice Ruins Access
Blast through an ice wall to encounter a Scatter Bombu. It's similar to the Pulse variety but it likes to block tunnels using its three streams of electricity. You can get through the streams without taking damage if you time it right, but you don't take a huge amount of damage even if you don't.
Ice Ruins East
The ground level of this room is guarded by two Baby Sheegoths. These are relatively tough for random enemies, at least compared to the ones you've seen so far. They are sleeping when you enter so try not to wake both at once. Their weak point is their back, so try to get to one side to get a good shot at it. Once the icy shell on their back is gone they will go down quickly if you fire from any angle. Meanwhile they shoot blasts of ice at you and if you're hit you'll be frozen in place. Press the 
button quickly several times to free yourself if this happens.
Follow the curve of the room around to the right; you pass the entrance of a small building but there is nothing very interesting inside. Up ahead are some ledges with crates which may be very welcome after your encounter with the Sheegoths. Scale the ledges, then turn left and scale a few more to reach rooftop level. Follow the ledges and platforms to the left and eventually you land on the roof of the building mentioned earlier. Pass through an arch of ice and stone, clearing the Crystallites as needed, and jump to the ledge on the right where a door is waiting.
Plaza Walkway
This passage is guarded by Ice Burrowers, basically cold adapted versions of the Burrower you met in Lake Tunnel. This is the first of several Ice versions of creatures you've already met, but their scan are still counted separately.
Phendrana Shorelines (cont.)
You're now on a ledge high over the Phendrana Shorelines, in other words you've gone all that way just to get back to where you started, but in a place that you can't reach otherwise. Don't fall off the ledge or you'll need do to the whole trek over again.
Follow the ledge to another door.
Ruins Entryway
Dodge the Bombus to get to the door on the other side.
Ice Ruins West
Before doing anything else get a scan of the Ice Shriekbats. They're in an alcove above and a bit to the left as you enter. This is probably the easiest scan in the game to miss; you can only get it this one room and in a normal playthrough you'll only pass this way a few times before the Shriekbats disappear forever. Not only that, but since they look and act like the normal Shriekbats you've already met many times already, you probably wouldn't realize that there was a new scan entry here unless you were following a guide.
There are Baby Sheegoths asleep on the left. You might as well wake them up and fight them since there's no way to sneak past them to get where you're heading.
Just to the right of where you came in there is a doorway into a small building. Go inside and scan the inscription there.
Go back to the entrance and go straight ahead, bear right at the fork and follow the right passage. You pass another doorway but there is nothing interesting inside. Fire a rocket to clear the door, and go through. There is more to explore in this room at rooftop level up it's unreachable for the time being.
Canyon Entryway
Weave past the Bombu to the other end of the tunnel.
Phendrana Canyon
Drop down to the metal walkway, then turn back and scan the inscription in the little cave where the walkway starts.
Go to the end of the walkway. There is a Baby Sheegoth below but you may be able to get a shot at its back without dropping down first. Once you're under the walkway there is a U-shaped passage in front of you, but there's nothing you can do there yet. Go up the slope on the right to find a small panel in the shape of a Chozo. Go right, behind the wall of ice if you want to get back to the walkway for some reason, but you're supposed to be looking left. There are hovering platforms ahead but they're at the wrong levels. Scan the panel to align the platforms, then hop from one to the other until you reach the tower at the far end of the valley. Don't dawdle on the platforms because they disintegrate soon after you step on them; if this happens then get back to the panel and scan it again.
Get the Boost Ball in the tower. There's nothing more to do here so it's time to leave; the game gives you a little preview of the route you're supposed to take when you get the Boost Ball.
Drop down into the the U-shaped passage. The game calls this configuration a half-pipe and perhaps the main use for Boost Ball is the the half-pipe jump. Roll to one side of the U, then start a boost and roll back, releasing the energy just as you pass the lowest point. If you did it right you should go a bit higher this time. Start another boost and release it at the lowest point going the other way to go even higher. It's best to just let gravity and the boosts do the work, so don't use the control stick. Also, start the next boost as soon as you can after a release; that way you should be up to full power on the next swing. Keep pumping this way and you can get quite a swing going. Note that the camera positions itself to give you a good vantage point for this.
It takes some practice to master this maneuver, and this is as good a place as any to start. You use it several more times in the game, but it starts easy and gets gradually harder so by the end you should be an expert.
Start around the middle of the half-pipe and start swinging until you're above the ledges opposite the platforms you used to reach the tower. (The camera can be pointing in either direction here, so it's impossible to say if the ledge will be on the left or right.) Give it one more full swing with a bit of extra welly and push the control stick toward the ledge at the top to land on the ledge.
Switch out of Morph Ball and follow the ledge up to the end, then drop down to the door where you came in.
There are three more half-pipes you've seen already; two in the Chozo Ruins and one in the Tallon Overworld. There's nothing more to do in the Phendrana Drifts or the Magmoor Caverns for the moment, so it's time for some backtracking.
Expansions (Chozo Ruins)
There's no need to go through the Ice Ruins East getting back; just drop off the ledge in Phendrana Shorelines when you get to it. As you enter Phendrana Shorelines there is a short cutscene where you see a shadow moving across the valley and look up to see Meta Ridley flying overhead. When you get to Monitor Station you can either go to Monitor Tunnel which leads to the Chozo Ruins, or you can go through Shore Tunnel which takes you the Tallon Overworld via the shortcut mentioned earlier.
Main Plaza
Get back to the Main Plaza; you may notice that the Beetles you met when you first arrived here have been replaced by a Plated Beetle. It's much easier to defeat now that you have the Charge Beam and usually drops an Ultra Energy.
Get to the U-shaped passage which you will now recognize as a half-pipe, and locate the Missile Expansion in a nook under the bridge. Now just do a half-pipe jump to reach it.
Ruined Shrine
Go to the Ruined Shrine where you picked up the Morph Ball earlier. Jump on top of the wall, but don't go over unless you want to fight another Plated Beetle. The wall is another half-pipe, so switch to Morph-Ball mode and start swinging. The camera always positions itself facing the way you came in, and you need to half-pipe jump to the right ledge facing this direction. There is something on the left ledge too but you need to come back later to reach it. Once on the ledge, remain in Morph Ball form and go into the small doorway to find a Missile Expansion.