4) Boom

Make a boom from 25 x 50 mm wood. Pinewood is great for purpose because it is strong enough and light. Use good quality wood without knots. The same wood you can be used for gaff.  

Leave other side of the boom straight. Straight side comes against the sail sleeve.

Measure position of the gooseneck (dimension X) when the gaff is attached to boom and sail is mounted.

Measure position of holes (dimension Z) for halyard and sheet line when gaff is attached to boom and sail is mounted.

Hole size of 8 mm is good for ropes.