I (and I think a lot of people) have a bit of a love/hate relationship with Reaper's envelopes.
For most basic tasks they're ok, but a lot of the editing functions are less than intuitive. The tools we have for drawing them are kind of basic. There is no easy way to draw periodic functions. And each envelope controls exactly one parameter and can't easily be routed or linked to others. We have workarounds for some of this, and some things coming soon that look somewhat promising, but the whole thing is just kinda wonky. But unless you're trying to use Reaper and a bunch of random plugs as a big modular synthesizer or like mixing a Hollywood movie, you probably won't run into those issues for quite a while.
You may already be aware, but Reaper kind of "wraps" every plugin you insert in a shell with certain controls that basically help define how Reaper interacts with the plugin. There's the preset stuff at the top left of the window, and the over to the right there are things like the Wet knob that allows you to mix some of the dry input from before the plug into whatever is coming out (parallel compression!) even when the plug itself doesn't let you. There's a button that usually says something like 2in/2out which is basically a patchbay that allows you to connect the plug's ins and outs to whichever of the 64 parallel track channels you want.
Then there's the Param button. Inside that are all kinds of things you can do with parameters including linking them to external controllers, Paramer Modulation, and setting up envelopes. All that to say that you can wiggle the knob you want an envelope for, click the Param button, choose "Show envelope", and it just pops up ready to draw.