Click on the specific board in question. Along the left hand side of the, between the number of the last 50 posts and the subject matters of those posts, you'll see a small picture of what looks like a push pin that we (old folks) used to use on a corkboard to hold a piece of paper up. Click on that pin and the particular post you would like to make a sticky becomes a sticky. You have to be a moderator in order to make a post a sticky or to remove one as such.