Once you've taken your hook and inserted it into the bait, make a loop in the line. Your hook will be at the bottom end of the rig through the bait (with the point of the hook facing up the trace line if you've got the position right) so the loop will be up the mainline away from the hook.
Put the loop over the bait - the past farthest away from the hook, and gently pull tight.
If the loop unravels it just means the loop is the wrong way around. Make a loop in the opposite direction and try again. When the loop sticks you know you've got it right.