You have to practice a little but it's possible. Feels pretty weird at first but that goes away quickly. Just use the tip of your tongue to try and get it as far backwards on the top of your mouth. If you reach far enough it feels softer, that's the start of the little bit of flesh hanging down in your throat. As soon as you are there don't be afraid to push up and the tip will go right above the bone in the top of your mouth, right into the nose. With practice you can find the entrances to you nasal side passages as well as stick the tip of the tongue into one of the two openings leading out of your nose. Stick a qtip in your nose and you can touch and wiggle it. Very fun to show kids. X)

There is a Wikipedia page with pictures: https://en.wikipedia.org/wiki/Khecar%C4%AB_mudr%C4%81