This scenario is fairly easy, if a little tedious.
A few seconds after the scenario begins, you are attacked by three fire ships. Depending on how fast you can move your hands, you may not have had time to do anything at all. Fire ships are deadly, but perhaps not more dangerous to your ships than your own Juggernaughts. It may be irritating to lose ships to friendly fire or anything else, but in fact, you could lose half your warships without diminishing your combat power in the slightest. To avoid losing ships, avoid your own fire as well as the enemy's. On the other hand, the fire ships will be sunk very quickly. After the first skirmish is over, make sure that your ships are in control groups, with the damaged ones left out and in the back.
Your immediate priority is to locate and destroy all enemy docks. You will not have any opportunity to convert them. As you explore the sea, you find that the Veneti live on their own island. Only the Veneti have a fleet. You will need to sink about 5 more fire ships eventually, blow up several Towers, and kill a bunch of archers, in addition to destroying their docks. Make sure that your Juggernaughts do not shoot wildly. You want to control where every shot goes. Set your diplomatic status accordingly.
The Veneti do have Transports, and will eventually invade you, so make sure that you sink them quickly. The Veneti are limited to only 20 units, so they will not be able to produce any new ones until they have lost a bunch.
The scenario instructions tell you to not bother converting villagers. This is incorrect. Your priests can also convert buildings, and one villager plus one Storage Pit equals a complete economy. Make sure that you actually get them! This is not essential for winning the scenario, but investing some time and effort, and especially thought, here will make the rest of the scenario much, much easier. You should also be able to take over several Cavalry units, which is very useful, because it is the only way that you will ever get Cavalry.
When the Veneti have been pacified, convert anything of theirs that you can use, and destroy everything else. There's not much space on their island, so eradicate everything that you can't use, including the walls. Keep in mind that Juggernaught fire wipes out trees.
Put your new villagers to work, and produce more if you can. Don't bother disbanding units just to get more villagers, because you don't actually know what you will need yet. You find that you will be able to produce Centurions but nothing else. Still, that's better than a kick in the teeth. If you can convert other enemy production buildings later, then you will be able to produce other units.
There is a lot of gold and wood in your new base. The immediate benefit of your new wood-food-gold economy is that you can repair damage to your ships.
Invading Britannia proper – Yellow
The time that you have spent assimilating the Veneti has no effect on the rest of the scenario. Your remaining enemies are all defensive. They will only act to defend their bases, but they have a huge resource stockpile available to replace casualties.
Start your invasion by blowing up all the Towers on the coast. Stay alert for enemy catapults and other threats, although the effective threat is small, now that you can repair your ships.
There are two invasion points. One is a good beach. A river runs inland here. Sending ships upriver is risky at first, because you don't have any maneuver room. The main problem with this beach is that it is far away from the enemy camps, so your invasion force would need to operate far away from the protection of your fleet. The second beach is very small, although it is close to the Yellow base. Start your invasion at the small beach.
Yellow has a temple, so you can expect to encounter priests. Unfortunately, you have no anti-priest weapon, except for the catapults. Therefore you need to spearhead the attack with a catapult. In short order, you find out that Yellow responds to every attack with Scythe Chariots and chariot archers. Since these units can be produced forever, you will never be able to advance far from the coast. You need to develop a better strategy.
You may know that the Romans augmented their heavy infantry formations with auxiliary troops. You are going to do the same here. You simply must have a force of chariot archers, and you probably want some of those heavy chariots as well. You cannot expect success if you go up against massed priests without a lot of chariot archers. Chariot archers are also some of the best scouts in the game. Aim for an initial complement of 5 each. You will find out soon enough that having only 5 chariot archers is just not adequate.
Capturing chariots is very hard, but however hard it might be, it is not as hard as trying to proceed without them. There is a very specific procedure that you need to follow. First, remove all artillery from the area, as well as other troops that might attack the unit that you are trying to convert. Stand right on the shore with both of your priests, with a Transport available right behind them. If things get hairy, your priests need to be extracted at a moment's notice, and you should know that getting into a Transport in AoE doesn't always work seamlessly. Lure the target unit into range of your priests somehow. The target unit needs to be entirely alone, with no friends anywhere in the vicinity, otherwise this procedure won't work. When the target unit is in range, convert it with BOTH of your priests simultaneously. The target unit goes catatonic and cannot attack either priest. Wait for it to come over. With chariots, this could take a very long time.
Once you have enough chariot archers, start exploring the map and raiding Yellow. You could go for the Brigantes and Picts, except they are so heavily fortified that you will need siege equipment. However, you will not be able to deploy it inland until the Yellow cavalry is eliminated. So you raid and raid and raid, and eventually, eventually, Yellow runs out of troops. Then you can push your chariot archers right into the Yellow base and kill all their priests. Expect to lose some chariot archers in the process. Think about how bad it would have been if you had tried this with other troops.
It's interesting that Yellow runs out of troops here, because there still is no shortage of wood in their base. While they have in fact used up all their gold, their woodcutters have only got lost somehow.
When Yellow has no troops left, blow up their base, but convert anything that you could possibly use. At this point, you could produce all sorts of units, and you have the economy to allow it, but your force is now so powerful, and your remaining enemies so weak, that there isn't really any need. In fact, the Brigantes have probably already thrown all their mobile units at you, leaving them with nothing but static defenses.