Dressing for Pleasure, Access Denied and Neosteel have a measuring form with very precise notes describing in each case exactly what you are measuring and how to do it. Make sure the measurements are accurate. Double check everything. It is impossible to engage in regular intercourse while wearing one of these. As for getting relief in these, accounts vary.
Some men report that they are able to achieve what amounts to an orgasm without ejaculation.
I've personally never had any luck getting satisfaction while wearing mine. The female account I have reports that it is possible to get a finger or two behind the shield for manual stimulation. The lock rests behind a steel flange. There is no way to get to the lock to cut it off. The waist band is 1.25 inch 16 SWG steel.