I find that attaching the shackle can be tricky if you have no slack in the stays. I'd recommend you take some tension off one of the stays (refit to loosest setting), then raise the mast and attach the forestay. Then add tension to the sidestay (with one of you using a trapeze line taken towards the back of the boat and hanging from it).

My Dart 18 and Spitfire both use a line to tie the forestay to the bridle shackle. This avoids the tensioning problem, as your crew can loop a few lengths of line through the shackle to hold the mast up, then you can adjust tension as required.