Having bs 5-0's and tails will help you a lot here. Get the lock in correctly down first, it's strange and awkward when first learning. Try going slow and just stalling them at first, or even just posing one on the ledge to get an idea of how your weight should be. Once you are comfortable locking in, try to grind them by leaning back, way back. You want your head over your back foot. Front foot just keeps the board pointed down. When locking in try to just place the truck on the ledge instead of jamming it up there (if that even makes sense). For me I find going faster helps, but not so fast you have no control. I try and focus on pushing down on my back foot with it about right in middle of the tail. It will take a lot of trial and error. I have my days where I do them every go, and others I land one or two on the sessions I try them. Nik Stain and Cyrus have my favorite ones right now. Watch their footage to get stoked.
Another pal had some good tips in the tricks you're trying to learn thread, think its within the last few pages.
Hope that helps