The water being cleared, you have just "finished" the Chozo ruins site. Of course there are still dozens of things to do here, but with the Varia suit in your possession, you can now withstand the heat of the Magmoor Caverns, the highway of Tallon IV. Coincidentally, the door just opposite you happens to open on an elevator which descends to Magmoor.
Proceed to the Sun Tower Access.
Sun Tower Access
Scan: Pulse Bombu
The only thing interesting in this room is your first encounter with a Pulse Bombu, which happens to currently be invincible, since you don't have the Wave Beam. You may be tempted to spend hours crawling around in the water under the ledge looking for secret upgrades. Don't. To the best of my knowledge there's nothing there.
Proceed to the Sun Tower.
Sun Tower
Scan: Chozo Lore: Contain
Someday you'll roll down. Today you can say "ouch." Just jump. Kill the bugs, scan your scan and proceed to the Burning Trail.
Path: Transport to Magmoor Caverns North, Transport to Chozo Ruins North, Burning Trail, Save Station Magmoor A, Burning Trail.
Burning Trail
Scan: Grizby
Proceed to Lake Tunnel.
Lake Tunnel
Scan: Burrower
Proceed to Lava Lake
Lava Lake
Scan: Magmoor, Puffer
Follow the islands around the edge of the room. At the end of the path, are a few crates blocking a sandstone wall. Use a bomb to remove the wall, opening up a tunnel with a similar wall at the end.
Proceed to the Pit Tunnel
Pit Tunnel
Scan: Triclops
Proceed to the Triclops Pit
Triclops Pit
There are two possible paths in this room. Resist the urge to proceed straight forward across the platforms.
To the right as you enter the room there are a group of crates on a ledge. Jump down the ledge onto the false floor. Around to the left there is a recessed pit. Enter the pit, turn around, and enter the true floor through a morph ball tunnel. Careful of the Triclopses, follow the path to the other morph ball tunnel.
Proceed to the Storage Cavern.
Storage Cavern
Fairly Simple. Follow the path, pick up your missile upgrade. Proceed to the Monitor Station.
Path: Triclops Pit, Monitor Tunnel, Monitor Station.
Monitor Station
Finally, some real action! Immediately, before you do anything else, there are two missile turrets you can fire on before you are close enough for them to notice you. A few steps in, and you can target a third.
Following the path around to your left, there is a door. Don't bother with it. The only place interesting you can get to that way right now is your ship. You didn't come all this way to go back to your ship, did you?
Proceed up the ramp to the structure in the middle. Then across the platform to the ledges ascending the edge of the room. Ascend, keeping an eye out for defense turrets. Continue around until you can jump to the catwalk back to the second level of the central structure.
Proceed to Transport Tunnel A.
Transport Tunnel A
There's an energy tank here, but describing how to get it is tricky. It involves doing the double bomb-jump trick directly over crates that disintegrate after two bomb bursts. The first crate as you enter the room (which, incidentally, hangs over hot lava) is the beginning of the path. Fortunately, I don't need to describe it in too much detail, because the game demo actually shows Samus completing most of this expansion for you. If you can't figure it out, restart the game, and sit through the demo a few times. If you still don't get it, you may have to have somebody else do it for you.
Proceed to the Shoreline Entrance of Phendrana Drifts. It's time to come up for air.
Path: Transport Tunnel To Phendrana Drifts North, Transport Tunnel To Magmoor Caverns West, Shoreline Entrance.
Shoreline Entrance
Blast your way through a few ice walls and proceed to Phendrana Shorelines.
Phendrana Shorelines
Scan: Crystallite, Flickerbat
As you can see, the Shorelines are something of a hub for Phendrana. There are six exits, including the one you just came through. You're going to be returning here a lot. For the present, however, directly across from you is the door to Save Station B. Just use it. It's the right thing to do.
Coming out, you need to go through the only door in the wall to your right. But if you attempt to address it directly (there is a series of ledges) you find it is artificially locked with the kind of blast shield not to be addressed with simple missiles. The console to unlock it is in the protruding tunnel just beneath. There is a Grate in the way, but it 'can' be blasted with a missile. Go over the snow drift as a morph ball, and scan the console. Now through the door above.
Proceed to the Ice Ruins Access.
Ice Ruins Access
Scan: Scatter Bombu
Blast through an ice wall, and scan another kind of Bombu, then proceed to the Ice Ruins.
Ice Ruins
Scan: Baby Sheegoth
This room is moderately simple. You need to proceed right by going left. First take out the Baby Sheegoths. Then follow the curve of the room until you see a stack of crates on the only ledge you can jump on to. This is your break. Follow the ledges and rooftops around to the left until you've put the next exit directly to your right.
Proceed back to Phendrana Shorelines via the Plaza Walkway.
Shorelines Redux
Same room, higher up. Now go higher. There's a door to your Left. Proceed to the Ice Ruins West via the Ruins Entryway.
Ruins Entryway
This time I'm afraid you'll have to proceed left by going right. Go straight ahead (past the Sheegoths - there's no avoiding them) toward the left side of the room. A hard right leads to your door.
Proceed to Phendrana Canyon via the Canyon Entryway.
Phendrana Canyon
This room looks ugly, but it's really simple. The Boost Ball is high up on a ledge at the far end of the room. There are hovering platforms all the way across, but they're at the wrong levels. Half-way up a slope on the right is a terminal. Scan it, and the platforms will align. Follow them and you've got the Boost Ball.
Wasn't that a long enough journey?