When you've created a fitting band and you're ready to test out cups, then you stitch them together and sew them into the fitting band. The following images are from directions for the Eve Classic bra cup pattern, used with permission.  These instructions are for assembling the whole bra, so there is a couple steps I'd leave out for a fitting cup that are featured in these diagrams.   You can use this same principle of making fitting cups (or test cups) with any pattern to achieve the best fit. Once you've added the cups, you can move on to assessing fit and making pattern changes. 

First, If you're using the Eve Classic pattern, stitch together the lower cup.  

The pattern instructions have you press the seam open and top stitch with a seam tape.  This is a great step for a finished bra, but not necessary for a test bra. Then, put elastic on the neckline edge and the armhole edge so you can baste it into your fitting band for fitting.

I wouldn't waste expensive, fancy elastic on this step.  You could just use 1/4 inch elastic and straight stitch it on. If you're in a big hurry and want to make the cup construction more simple, you could skip this step, but then you'd want to off set the cup from the bridge by the amount of the seam allowance, and then do the same in the armpit area so that vertical seam attaches to the base of the fitting band in the proper location.