It's really hard to describe. But it's not simply a matter of up and down really. If you simply move the tongue up and down, the air flow, direction and volume is constant. It's both changing the shape and expansion/contraction to change not only the trajectory of the air but also the internal volume of the mouth. If you can speak or drink through a straw, you probably have the tongue dexterity required.
Try this. Imitate the sound of a wind storm, vary the sound from a fast wind to a slow. In doing this the volume of your mouth will change and the pitch will change low to high. Your tongue moves forward and back, expanding and contracting. This sort of motion should also result in slight timbre changes. Move on to your throat, expanding as if yawning, then contracting back to normal. This will also have effects.
If this still doesn't work for you, one on one with a teacher is what you'll need (whenever that's practical again). He/she can evaluate any physical limitations, see/hear what you're doing and give more specific instruction.