It's not that difficult, really. I'm finishing up a grand campaign in 1815 as France, with most nations (including Austria!) allied with me.
Basically, my main targets are the minor European states. The great powers - Britain, Prussia, Austria, Russia, and Spain - will more or less always hate you, outside of scripted events. You can, with effort, keep them neutral, but it's mostly not worth the bother (only Russia seemed amenable, ever). Instead, focus on the little Italian and German princelings in-between - Bavaria, Baden, Tuscany, etc. Send state visits whenever you can, follow up with commercial agreements and supply exchanges nad so on. With patience, in a couple of years you'll ahve alliances with most of the minor states of Europe with no real effort needed beyond this.
Outside of Europe - the minor powers in, say, the Balkans, the Middle East, India, or the Americas - I didn't really bother. Didn't seem to get much return.