This how I occasionally fap. I did MS Paint so that you guys could understand better.

Ok this is how I do it:

First of all you need a sink that is at your penor's height otherwise it can be quite tricky and difficult but it is possible, it might be harsh on your leg/butt muscles. Make sure you lube everything, you hands and your penor before you start otherwise it might hurt. You'll probably have to add lube/water as you go along, I've noticed it gets quite dry after only a few minutes.

Step 1: place you penor on the sink and find a good stance, not too wide, not too narrow. I like to be a bit more than a shoulder's width. You can always change afterwards.

Step 2 (see image): place your both hands/fingers so that you create a kind of hole with the sink and your fingers. Make sure it's not too small otherwise it won't glide smoothly, make sure it's not too wide otherwise you won't feel a lot. You can change your hand stance later on if you wish. There can be a lot of different positions, different positions give different sensations.

Step 3 (see red arrows): start moving your hips/body so that you penor glides through the hole created by the hands and the sink. Be careful, if you are a beginner, you need to find the right hand positioning so that it doesn't hurt your penor and especially the skin underneath that's in direct contact with the sink. I always watch some porn on my phone while doing this, I place it above the sink on the shelf.

In the beginning you might need to use your hand to bring yourself to the orgasm.

The cum goes inside the sink, don't have to worry about cleaning it up with tissues, etc.... you just clean the sink, I always rub it with soap and rinse it completely.

There, I hope I've been clear enough, the MS Paint should help.
Try it and report back.

p.s. Close your bathroom door while you do this.