Let's start with some general tips useful for each Nation.

To win the game you need:

1) commercial harbours and commercial fleets.
2) exclusive possession of one or more commercial areas, especially in the Mediterrean Sea.
3) Propagantist Agents to win the support of the Spanish people.
4) Supplies.
5) The more advanced possible streets
6) Universities and their researches.
7) A strong economy, before than a strong army. You cannot rely only on taxes.
8) A careful examination of the land and of the movement options of your troops/ships. It could be useful to plan ambushes and deploy your troops/ships in the best way.
9) Armies with strong and powerful artillery as well as infantry and cavalry.

The British Army is quite strong (if not the strongest army), but it takes a bit to create it. Initially, you can start by enlisting also some Spanish and Portuguese troops, but you cannot rely on them too much.

Guerrilla units can be used to sabotage some of the isolated enemy's buildings and to attack unprotected areas.

It is capital to focus on strong harbour facilities rather than building exclusively military facilities, not only for the economic growth of your nation, but also to enlist the strongest units of the British Army and cut military costs.

Be sure to conquer a territory where you can start the cultural research!

Finally, you can easily reign over the seas, by using the Royal Navy and exploiting the four commercial areas.

The problems with the French are:

1) You start by controlling almost the whole Peninsula but you are already in debt, so you'll have to cut military costs at first. However, you must keep some military forces almost everywhere, because the Spanish can attack your furthest regions by using the Guerrilla forces. Be sure to exterminate them, otherwise they can hide in forrest areas and heel automatically after a short time.

2) You have to face 3 enemies at the same time: Portuguese, British and Spanish.

The Portuguese will threat the western costal part of your empire, not immediatly but after forming substantial armies. However, they are not very aggressive and quite predictable.

The British are the most dangerous enemies, because they can relentlessy deploy superb troops and they have a superior navy.

The Spanish can benefit from the support of the people and constantly enlist large armies against you.

3) At first you have advanced military buildings only in the French soil, so it will take quite a long time to transfer the best troops.
Moreover, some of them, like the Vistula Ulhans, the Swiss infantry and the Poland Legion, can only be formed in the France.
However, you can transfer those troops using a ship.

Positive sides:
1) Perhaps the best army in the Peninsula, but keep away from the militia!
2) The British don't take advantage on their navy in the Mediterrean Sea (but only in the Ocean), so you can build a commercial fleet and exploit two (out of the four) commercial areas.

Tips:
Propagantist Agents are extremely important to win the support of the Spanish people.
Use spies to check the movement of the enemy troops, to sabotage its lands and to cause rebellions (if the area already supports the French rule)
Develop all the military and civil buildings in Toulose and secure cultural breaktroughs as soon as possible

The problem with Spain are several:

1) Its initial regions are surrounded by the French who can send big and better armies.

2) Its army is not very good compared to the French and it is largely outnumbered.

3) At first, the Spanish have few resources, but they can profit from the help of the British.

4) Finally, if you play the Spanish you'll also have to wait a lot before winning the support of the other parts of the Peninsula. Don't allow your agents to duel the French ones at first, but only when you have already won the support of the people.

Positive sides:
1) Use of the guerrilla forces who can hide and heel in the woods.
2) Alliance with England forces who sometimes give you the regions they conquer.
3) Support from the Spanish people

Tips:
1) The priority is to secure an area where you can also use its ports and its education facilities, but it will take a long time to reach that stage.
2) Try to conquer a region with strong supplies.
3) Try to coordinate your action with the British forces used by the AI.
4) Win the propaganda battle with France with your own propagantist agents.