The simplest and one of the oldest systems for spanning a crossbow is the belt-hook. Historically, belt-hooks show up about the year 1200 in illustrations and literature. Belt-hook capable crossbows are fitted with a stirrup fastened beneath or to the front of the prod. The shooter is provided with a hook (preferably a double-hook) hung from a stout belt. (I prefer a belt at least and inch and a half wide, with a double-tongue buckle) To span your bow with a belt-hook: hang the hook in front of your crotch. Turn the top of the stock (table and lock side) to your belly. Hang the bowstring in the hook. While balancing on one foot, lift the other foot and put it in the stirrup. Push down on the stirrup with your foot, while guiding the bowstring/hook into the lock. Plenty of medieval illustrations show single-hooks being used for this purpose, but a properly designed double-hook will get the string centered more precisely. When the string is in the lock, carefully remove the bowstring from the hook and THEN remove your foot from the stirrup. Do it in that order… otherwise, a lock failure when the bow is no longer attached to your foot may allow the bow to snap-back, hitting you in the belly with the butt of the stock. With a lightweight bow that could really hurt. With a strong-bow, it could be deadly. There are medieval stories about such mishaps.
Alternatively, you can put your foot in the stirrup, crouch-down, and slip the hook over the bowstring. Then you stand up, guiding the string into the lock. It’s more work that way, but you don’t have to balance on one foot.