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Then take the other end and pull it through the other groove.  at the end to hold the string - then you're ready to shoot with your bow.

Summary:
Take a long tube of PVC, and cut it into a piece about 4-feet long (120 cm) for a short bow, or 6-feet for a long bow. Cut a small groove into the ends (to hold the string) with a hacksaw. Take your string (preferably masonry twine) and tie a loop on one end. Pull it tight, then cut the extra string off and leave room for a knot. Tie a knot