There are normally two big terminals, one directly from the battery, and one directly to the starter. Those two need to be shorted together. Usually it's close to the battery.
Usually they look about like this:
https://images.app.goo.gl/HTznwVrFX84NjqXZ7
Or this:
https://www.ebay.fr/itm/Relais-de-de.../151994467134?
Yes, they're usually covered by a rubber boot, but it should just peel right off.