I have been playing France for quite sometime now on M/VH and I have taken over entire Modern France, England, and Spain by now. I started out by taking rebel provinces making sure to have alliances with all of my neighbors and of course the pope. It is very important at the start of the game to avoid conquering certain provinces if you do not want to start war with your neighbors, for example whenever I conquered Antwerp I had the Danes attacking me even though we were allied. Same goes for provinces in Modern Spain, so try to avoid those at the start. My next step after conquering modern day France was to go for Britain.

The advantages of this strategy are many:
1. You only have to take Caen at the start and then the English cant attack
you because the AI does not cross the sea.
2. Britain is large enough to provide a decent income and the most easily defensible area of all, simply because there is no way the AI will attack it.
So if you capture it chances are you will keep it with and with minimal unit upkeep.
3. The English will probably already be engaged in a war with the Scots and they will be either occupied on two fronts or weakened from the conflict.

By having conquered Modern day France I was able to have about 4.5 full stack armies which I used as follows :

1. 3 Armies used to attack Britain one of which landed on Ireland and the other two landed near London.
2. half a stack was left guarding my borders with Spain
3. and the remaining army was left guarding my eastern borders.

With all the above armies I was left with a decent income with which to continue developing my empire.
I was able to conquer Britain quite easily because the English were already at war with the Scots and were losing. As a result I first eliminated the English and then destroyed the Scots making sure that I followed papal orders when he told me to take a break, which was actually useful at times because it gave me some time to regroup my armies and also my enemies usually did not hear papal orders and as a result were excommunicated.

Hint: make sure that you take Cagliari and the other island in the mediterranean and turn them into cities for a very nice and easy income and also you can use them as naval bases later on should you choose to go after Italy.

After I finished off with Britain I went after Spain with my Veteran armies from the British campaign. This was much more time consuming because for almost the same number of provinces (8 i think) Spain is at lease 3 times as big which makes it more difficult to obtain reinforcements and reach targets.

During my conquest of Spain a crusade was called in which I joined with a full stack army, but instead of rushing for Jerusalem and due to the fact that the Mongols had arrived I opted instead to take Cyprus first from the Byzantines because it is the ideal place from which you can proceed to launch raids on the entire eastern Mediterranean. By the time I arrived to Jerusalem the Danes had already taken the city. However using my now regular army I took over Acre, Antioch and then proceeded to Alexandria. Now I am preparing for war with the Mongols who have at least 12 full stack armies but seem to be stuck near Baghdad and wont do anything but sit there.

I look forward to trying a new technique against the Mongols where I will mostly use Serpentines against them in order to counter the fact that they mainly use archers. Also the best units I have used so far are the Scots guards which have the highest kill rate of all my units, including heavy cavalry, I am now trying to recruit Aventuriers which look like they can be even more effective. If any one has any tips on how to handle the Mongols please share your thoughts.