So, really, I had nothing to do with cutting the liners. I researched some options with Paper Source - both liners and templates - and was all set to use their system, until I found this paper on Zazzle, of all places!
Once Whitney got everything cut, I got to work with my favorite glue (another recommendation that is all thanks to Bianca - what would I do without her?).
Here's all you need to line envelopes on a clean, flat surface:
+ Yes! paste
+ small sponge (cut a scrap from the kitchen) or sponge brush
+ a small paper or disposable bowl, plus a few dabs of water
+ nice to have on-hand: a paper towel or two
Step one: stuff liners in envelopes and fold at the crease.
If you're doing a big project - say, 100+ wedding invitations - I recommend having paper custom cut by a company like Paper and More. I used them to cut liners for my rectangle-flapped wedding invitations, and then did DIY lining at home, using the above process.