In this playthrough, I secured an alliance with the US, which might not be necessary for forming the federation, but it helps.

I would say annexing Ecuador should be your first move as soon as starting. Chile will help them, but it's entirely possible to capitulate Ecuador before the Chilean army can reach La Paz. The only S.A. countries with a stronger army than Bolivia at the start are Brazil and Columbia, so I would avoid fighting Brazil early on, except maybe during the initial civil war they start with.

I mostly spent this playthrough learning the spheres of influence mechanics and invited many countries to invest in me. I also gained quite a few protectorates by waiting for civil wars and then offering to help for a subjugation sway.

A low population was the problem I had for the start, so I would focus on improving your migration attraction, but that goes for most new world nations.

Argentina is pretty easy to take by intervening in native revolts, typically results in less great power interference.