The hardest thing for you to get right at this point is choosing a good board! Seriously the reason your bows have broken is most likely due to board choice. Look on all four faces of the board, you want straight lines all the way round. If you do this you can forget your drywall tape. To be honest it's a bad choice for a wood bellied bow because it will increase the load on the belly causing more set and a slower bow.
Find a perfect board.
Make a tillering gizmo.
Read up on pyramid bows.
Practise!!!