Light ships, light ships, light ships. Control trade. As Spain you want to colonize all of Venezuela<->Mexico and nab other bits of theCaribbean trade node. Brazil and Africa are very optional. Anyone who controls those just benefits you, since they have to bring all of that money past Sevilla or the Caribbean anyway.You make sure all trade flows to Sevilla (Caribbean and Genoa are interesting for this) and then capture it all in Sevilla before it flows to Bordeaux. Whoever controls the Caribbean controls whether the new world money goes to Sevilla, Bordeaux, or the Chesapeake Bay and then to Bordeaux/London/North Sea. You control the Caribbean and control Sevilla and the northern countries lose out on a LOT of money.
DO NOT WASTE MERCHANTS COLLECTING AT HOME.At least, once you have viable alternatives to steer trade to home. They don't scale up much (at all?), and their influence steering trade is worth exponentially more. At game start Castille doesn't have many options, but as soon as you have access to Genoa/Caribbean, your merchants should be there.
During war you can task part or all of your light ship fleet with blockades of your opponents to improve your odds of victory, and light ships don't cost any diplo points to build unlike trade buildings.
Don't bother colonizing north of Florida at all. The Mississippi river basin has potential, but controlling historically Spanish provinces is more useful and limits your border friction/war fronts with other european powers. You can't send any of the Chesapeake trade to Sevilla, and you have a penalty to collecting money that enters the Bordeaux node because your capital is in Sevilla. Write off anything that goes past you. Focus your efforts upstream.
If you're not familiar with EU games, one mistake new people make is overspending on your military when you're not at war. Early on you should keep your land maintenance at around 10% or so unless you're concerned about an upcoming conflict. Be wary that when you raise your spending to 100% you'll still need a few months before you can use your forces on anything except complete primitives. A good way to lose a lot of warscore is sending low maintenance soldiers to the front lines in a european war.
In my Spain game I had a total of 10 big ships and 0 galleys on register the entire game. My force limit was maxed with light ships and my coasts were maxed on shipyards (+2 naval forcelimit). It shouldn't be possible to build shipyards in colonies because that gets pretty damn OP when you have diplo points to burn.
And then when you're making ridiculous bank off of the new world, you'll realize the ass that is the trade steering bug. Your merchants will randomly decide to steer trade to Chesapeake one day, and you won't realize why your economy is tanking until you notice.
TL;DR, Light ships, trade steering, low land maintenance = money.
P.S. If you want to cheat, wardec Mali/Native Americans/Swahili/etc. for gigantic sums of gold. This is a flaw that should be addressed by the devs soon. They don't spend their money like...at all. So the cherokee, for example, pull in like 300+ gold a year you can grab every truce period with 5 regiments. Mali takes a bit more work since the native americans get defensive penalties that Mali doesn't.
