if you're ever in a situation with a bank to wallie spot, those are ideal for getting the motion of the trick and really getting used to 4 wheels on the wall. i learned wallies first on this of all things:
straight up wallies are harder and seem to have more of a mental block. if you can find something ie a bench/wall/etc thats about knee height, you can learn them off those off the corner which will give you the feel of it. in my time of teaching people how to do them i find it goes in 3 stages; the first being the kinda sloppy back wheel bonk, the second being all 4 hitting most of the time, and the third being real confident with them and feeling/understanding the wheels rolling up and off the object.
as self serving as it is, heres some screengrabs of probably my favourite thing to wallie in the hopes it'll help in terms of positioning/where to put yourself:
it's generally a hard trick to try and explain or give a trick tip for, because it really is just a case of full commitment and bashing the fuck out of it, and i think thats where a lot of people come short. around knee height is ideal for a wallie, obviously you can go bigger with time but generally knee height or something that allows all four wheels to connect are great as shorter ones tend to do weird wallies where the front hits, goes up then the back hits and bonks up whereas you ideally want all four wheels hitting.