Choose your thirty to forty best strips to submit. You should only have to include black-and-white daily strips. The syndicates will likely ask for Sunday strips only if they like the dailies.
Put the strips on a sheet of paper for easy reading and handling. Make sure they fit on a standard eight-by-eleven inch sheet.
Make photocopies of the strips. The syndicates should never want the originals for first submission.
Write up a description of the strip. Tell what the main theme and purpose of the strip is. Introduce and describe all the main characters.
Decide if you should copyright the strip first. With most major syndicates, this is not needed. Still, signing your name to each strip and contacting the U.S. Copyright office never hurts.
Mail the strips. Most syndicates still insist that you submit by mail and won't accept submissions by fax or emailed images.
Wait for a response. The best first response you should hope for is one saying your strip is being considered for syndication.