Step 1: Materials
-- An article of clothing for your target
-- A needle
-- Thread of a color similar to that of your target garment
Step 2: Execution
Sew with a large running stitch, you don't want to permanently close the opening, just keep it closed long enough to cause a bit of consternation or confusion in your victim. When you've finished sewing, put the thread back through to the inside of the garment and clip off any excess thread to conceal what you've done as much as possible. Don't tie the thread off when you're done, that just ticks your victim off.