Weddingbee boards

I either used a standard white glue stick (the kind kids use in school) or scotch double-sided tape.  I like the glue stick because it's easy and quick (just make sure you have something underneath the edges in case you go a little over the edge) it doesn't leave a "wet look" like liquid glue would.  And I like the double-sided tape because it is less messy, I have a separate tape dispenser for the double-sided tape and just use it to cut off small pieces for the liners.

I used one of those permanent adhesive rollers-- very quick and easy!

One tip I would have would be to insert the liner, and make the fold where the flap goes with a bone folder.  Then, with your liner folded down, put the adhesive on the three sides of the liner.  Then, fold your envelope flap down onto the liner, so it sticks, and press out from the center.  This worked much better for me, rather than putting on the adhesive and then folding the top of the liner back up to stick to the flat flap.

Does that make sense?  Had some bubbles/creases when I did it the other way, before I learned the secret.

I agree with jhphi completely on working on them while already inside the envelope.  I did mine a bit differently in that I flipped the envelope over with the flap flipped up, but it worked MUCH better with no creases.  I tried to do a tutorial for a friend on my blog with some photos that might explain it a bit better.  It's about halfway down the post:

http://craftysmurfette.com/2009/09/20/envelope-liners/