Here are instructions for how I like to frame the 24x18" paintings on paper that I sell here.
First of all, I choose a frame sized 30x24 inches. This size works perfectly if you float the painting inside a cut mat board that shows all the edges of the painting. The Michaels stores in my area carry frames in this size.
Please use archival mats both behind the painting as well as for the top mat. The painting will be "floated" with linen tape to hold it in place. Just fold the tape so that it can be slipped behind the painting to hold it invisibly to the back mat board.
The measurements for the upper mat are 2 5/8 inches from each outer edge. This will create an opening that shows off the painting with no masking off of the edges.
It's as easy as that. I hope my little primer will help you with framing the work. This is an easy job for an experienced picture framer.