Eventually you will get to the stage when your colonies are pretty
well self-sufficient and clamouring to be independant. Now is the
time to build a fortress in each, make sure they have plenty of
horses and guns and have as many dragoons and artillery as you can
manage.

Declare Independence and you will be attacked by the mother nation.
The other Europeans, if you've treated them well, will come to the
rescue. If you manage to withstand the onslaught, you will have won
the game.

These, of course, are the bare bones of gameplay - I am sure you
will think up refinements as you go along. Or you may choose to
take on the lot and end up dominating the whole of the New World.
Good luck!