This is a lot of fun and surprisingly I haven't been caught yet seeing how much evidence there usually is. It's easier to do when you're home alone and you 'slept in' when you usually do your bedding so no one is home and you have to put your bedding in the wash anyway.... just try to keep it on the stuff you put in the wash.
It's actually really simple.... take off your clothes and screw your bed. Just be carefull not to be right on top of your penis because the friction tends to hurt then. I have found that it feels better on different sheets but I don't wash those so I haven't come.... just got as close as possible.
If you really can't wait until the right day just keep it hidden for a while... it will dry and then if you rub the spot it will break up and you can barely see it and it won't be hard to the touch.