I would not build lather in the same container your soap puck is in because you will be continually adding soap to the mix while you are building the lather. Then you need more water, then you get more soap, then you need more water and you go round in circles. Load the brush with enough soap then go to a separate container to build your lather and add water as needed.

Whatever container you have your puck of soap in, that is where you load the soap onto the brush. Then you build your lather either in another bowl or directly on your face.

To get a thick rich lather you need enough soap on the brush with the right amount of water added. It takes a bit of practice to get the right ratio to give you the lather you want.

With a cream, as opposed to a soap puck, you can build the lather in a container by dropping a bit of cream in the container and having at her with the brush. You add just a bit of water at a time till you get the lather you want. The other way is to put a dollop of cream on your damp brush and brush it onto your face building the lather as you go and adding a bit of water at a time to get the lather you want.

Bob