Assuming that you have mastered the fully-relaxed erection induced by nipples alone, here is an enhanced process, particularly suitable for those in some kind of orgasm-denial/chastity. It can be supervised of course, to check that your face, body, and breathing are relaxed.

Create a full erection, then stop all stimulation immediately. The penis will slowly subside. (As a guide, for me the flaccid-erect-flaccid process takes around 3 minutes).

Now, repeat the process a set number of times. I have done 10, but find that 5 is enough. At this stage, the desire will be very high, so try to soak it up and enjoy it.

A discipline touch might be added occasionally, by a dominant - a tiny weight (e.g. a small towel) can be placed over the penis, so that it struggles to become erect. The weight must be light enough to allow an erection sometimes. This can be very frustrating.