Reps and Sets
It is ok to feel your muscles working by the end of a set, so long as you can maintain smooth, controlled movement. Holding your breath, straining, shaking or tensing other muscles (such as shrugging your shoulders) are all signs that you could be overdoing it.
As a guide, start with one or two sets of 10-15 repetitions. Increase the number of reps, add an extra set, or progress to the next option if you are finding it too easy and you are consistently able to engage your core and pelvic floor throughout.