You can do it sitting down - the conventional position. In this mode there will be a little bit of liquid dripping down, just like pussy juice. You can pad dry the jelly before use, but basically it's jelly and the surface will liquidize, though slowly. In the heat of the moment you will squeeze more juice out of it.
This is not a problem if you do it in the bathroom. But inconvenient if you want to sit in front you your big HDTV and watch your favorite star. It is not at all bad because a water proof mat or sheet will be sufficient to protect the floor, carpet and furniture, as long as you don't play too rough.
The can is very handy to hold with one or both hands. If you hold the can upside down, the whole block of jelly can come down if there's nothing below to stop it. You hands and your cock usually can prevent that happening but if you play rough, it's not hard to drop the can of jelly.
Ideally you find some sort of lid to cover the opening but with a big hole in it. So you cock can get through unobstructed while the jelly do not drop down when turned upside down. I think there are plastic can covers around.
The jelly provide a very good massage, a very good suction, and you can even ram hard on it without destroying it. All depends on how you make the hole.