Feet for clawfoot bathtubs are all slightly different designs depending on the manufacturer. Most have threaded studs already fitted to the bottom of the tub. The stud passes through a hole in the top of the foot which is then tightened to the tub with a nut and lockwasher of some type to stop it coming loose. Others, you will have to screw the stud into the tub first, in which case be careful not to overtighten it and strip the thread in the bottom of the tub - it will be expensive to repair.
I have installed several of these tubs for my customers in the UK and it's never easy. Although it's easy to fit the feet with the bath standing on end, or upside down, final tightening and adjustment has to be done with the bathtub in place, which usually means lying down full stretch on the floor and reaching under the tub to tighten the nuts.
If the floor is uneven - it usually is - it's necessary to use a few washers to "pack out" the foot from the tub to provide some adjustment.