I haven't created work in this way (yet).
However, if I was printing it with real handwriting -- if I didn't have the actual legal pad paper stock (pre-lined with margin/writing lines), I would use a suitable tint of process yellow (double sided), and add in relevant notepad and margin lines along with the handwriting.
I'd be inclined to do the handwriting on white paper (with a page paperclipped behind with thick lines as a guide) -- scan in the handwriting, drop out the background and superimpose over the yellow background with lines (that of course match the guide I used for handwriting).
The Bill Glazer one I have in my swipe is from a campaign selling the 2006 Glazer-Kennedy SuperConference (pretty sure Mitch Carson supplied the little planes for lumpy mail) -- came in an envelope with a very large window face showing the plane and free airfare offer (addressing on the other side). This was a "special late registration offer" -- see pic attached.
In this case, they used a font to mimic handwriting with some added squiggles, crossed out bits, margin notes etc -- and printed directly onto lightweight lined legal notepad paper using blue and red ink.
