Base chain double crochet is useful for more than just starting a project. Here you can learn how to use base chain double crochet to make a button hole.
When you are on the stitch before your button whole will start, yo and insert your hook into the next stitch, pull up a loop. Yo and pull through just one loop on the hook to prepare for the base chain double crochet, yo and pull through two loops twice to finish dc. Start base chain double crochet: yo and insert hook into the extra loop you made on the previous stitch, pull up a loop, yo and pull through one loop, yo and pull through two loops twice (you can also see the base chain double crochet tutorial)
Continue working base chain double crochet until you have as many stitches as you need for the size button you are using.
To rejoin to the main fabric, yo and insert your hook into the extra base chain loop on your flap, then insert the hook into the stitch you are going to join the flap to (in my case, I skipped four stitches and joined to the fifth stitch on the main fabric because I did four bcdc for the flap). Pull the loop up through the stitch and through the bcdc loop, yo and pull through two loops twice. Continue on with your double crochets!