No need to take the neck off - you can if you really want to, but it's more trouble for no benefit.
One possible method is to sit in a chair, with the bass body resting on the floor between your legs, and the neck pointing upwards. The bridge is against your right foot, and the neck plate is against your left knee. Use your legs to hold the body stationary, bend the neck with your left hand, then snug up the nut with your right.
Alternatively you could use a kitchen table or other work surface, along with some straps and/or clamps.