Go ahead, knock yourself out !!! The way I suggest is to begin by checking your surroundings to find out if you are where you are supposed to be, doing whatever you are supposed to be doing, such as your job or home. Then check your memory to see if you can remember the events of the last few minutes, how you came to be where you are. Then do a digital clock, writing, or number doubling test, or look for weird objects, or pinch your nose and try to breathe through it. If you conclude that you are not dreaming, say aloud: This is NOT a dream. It's important to bring closure to the RC by actually saying the result aloud.

The entire procedure takes a few seconds. I don't think it's necessary to overdo it. Just check yourself out like you look at the instruments on your car's dashboard to make sure the engine is running properly. Are you where you expect to be? Is your memory working okay? Do printed numbers or letters remain stable if you look away then back to them? Is any weird going on?

As far as the 90 minute time interval,do you really know if that is YOUR unique time period between dream? If not, your timing is almost certainly incorrect. I think it's better to do RC more often. I have a clock program on my computer that chimes every 15 minutes throughout the day. Although I haven't tried this yet, a simple way of triggering a RC during a dream may be to simply leave the clock going so that the chimes come on during the night. Set up a conditioned reflex.