To make a a simple, medieval-style crossbow trigger mechanism, let's use the example of a bow stock 6 inches wide. Just modify the dimensions to fit your own stock.
Use the wood chisel to cut a rectangle 1 inch wide by 1 1/2 inch through the crossbow stock (top to bottom) at the point you wish to mount the trigger.
Using the drill and 3/8-inch wood bit, drill a hole through the centre of the round side of the hardwood cylinder.
Using the narrow blade wood saw for the rough cut and the chisel for finishing, cut notches for the bow string and trigger into the hardwood cylinder as shown in the drawing.
Using the drill and 3/8-inch wood bit, drill a hole through the sides of the crossbow stock to mount the trigger block as shown in the drawing. Slip one of the metal rods, 3/8 inch by 6 inches, through as the trigger block pivot.
Using the drill and 3/16 inch metal bit, drill two holes in each of the 3-inch long metal straps: one being 1/4 inch from one end, the second being 1 inch down from the first and both being in the centre of the width of the straps. Using the 3/8-inch metal bit, drill a third hole 5/8 inch from the other end of the strap and centred.
Clamp the 5-inch strap of metal in the bench vice with the last 3/4 inch of one end held in the jaws. Bend the strap so the last 3/4 inch is 90 degrees to the rest of the strap.
Epoxy the remaining 3/8 inch metal rod to the 5-inch metal strap at a point no more than 1 1/2 inches back from the 90 degree bend and perpendicular to the length of the strap. Allow epoxy to harden completely. If you have access to welding equipment, welding this pivot arm would be better.
Fit the angled end of the 5-inch trigger bar into the bottom of the trigger block (see drawing from step 4). Use this positioning to determine the best point to mount the smaller straps to the crossbow stock (using the wood screws) to allow the 3/8-inch holes cut in the straps to hold the trigger pivot out at a comfortable distance from the stock. When a string is pulling against the trigger block, the trigger bar must hold the block in place. To release the string (fire the crossbow), the archer will squeeze the trigger bar up (from a grip behind the trigger pivot) which will pull the trigger bar down and out of the trigger block notch. This must be a smooth and comfortable action for the archer to ensure the best results.
There are a number of medieval weapons books with period artwork or engravings. One such is "Weapons & Armor: A pictorial archive of woodcuts & engravings" by Harold H. Hart; Dover Pictorial Archive Series. Your nearest chapter of the Society for Creative Anachronism or other medieval recreation groups will be a good resource for crafting tips.
When drawn, the crossbow string has a lot of kinetic energy stored, so be careful to avoid accidentally releasing the string. Always examine the trigger block before using the bow; a cracked block may fail and cause an accidental release. Use safety glasses when drilling metal or wood.