Step 1: Make sure there's no air in the tire. Lay the tire on a flat surface, rubber side down, valve at the top fold, and inside edges touching as shown. The rubber bands shown here are just to hold things together for the photo.
Step 2: Fold the bottom up toward the top, but stop short - near the end of the valve. The glued base tape will now be pressed against more glued base tape so no glue will get on the tread.
Step 3: Fold up about 1/3 the way from the bottom, and then fold up again toward the top.
Step 4: Secure the folded tire with a shoe lace or toe strap; it's ready to install under your saddle.
When done properly, the valve will have no stresses on it.
A tight and tidy tire tucked under your saddle is the mark of a sew-up connoisseur; take a moment to admire your handywork.
My Eddy Merckx Corsa has sew-up wheels in the summer, and tuff clinchers in the winter.