Hi Niels,
every plugin is different so these are the typical steps I go through when investigating a plugin that looks interesting;
step 1 - understanding the plugin, how it works, what it requires from the server and so on
First read the docs for the plugin.
Bring up a sample demo page with the plugin in it. save this html, and reduce it down to it's bare minimum.
Once you have the plugin working in a static page, and you understand what the plug needs then
step 2
add the .js file for the plug to the external scripts.
manually add embed code in appropriate places to get the plugin embedded onto a form, browse, source, menu or whatever.
In the case of a menu consider how the links are generated to support page mode, content mode and popup mode.
step 3
if the plugin requires async data from teh server (like say AutoComplete did) then that has to be handcoded into WebHandler.
step 4
if the plugin does any kind of "saving" then those calls have to be managed in WebHandler as well.
step 5
Once all the embed code is working, and I'm happy that it's compatible in as many modes as possible, then I create template code to generate the embed code, removing the embed code as I go.
Hopefully that takes you less than a couple minutes to read. Unfortunately it can take me half a day to do step 1, which is the longest, hardest step, so it's difficult for me to really assist more with that. Obviously the more complex the plugin, the more effort that step can take. Some plugins have taken days to get to grips with.
In many cases I've also found the quality of the plugin to be a bit spotty. So while I've certainly used plugs (the HTML uploader and Signature plugins spring to mind) I've also had to write quite a lot of JavaScript to make those "work right" in the variety of cases we support. For example I need ThemeRoller support in all the plugins and so on.
Cheers
Bruce