First, you want to put the forestay in the top hole of the chaimplate in the bridle. (With the mast up) if the shrouds are slack, then have one person hold one shroud, then the other person can remove the pin from the shroud chaimplate and move the shroud thimble down as far as possible and reinsert the pin. Then do it to the other one. That'll rake the mast as far back as possible.

If you have the shrouds in the bottom holes and it's still slack, then for now you'll just have to put the forestay in a lower hole. Don't worry, everything will tighten up when you raise and tension the jib, as the jib wire takes over the forestay's job once it's tensioned, and the forestay will go slack.