Step-by-step: Get some perspective
Start by loosely drawing the amount 1 of foreshortening you want in a sketch from either your mind or reference, but don’t use perspective lines. Just mess around until you have an amount of exaggeration that expresses what you’re going for with the image. Now figure out where the horizon line 2 and vanishing point would be. Do this by either finding parallel edges on an object (like around the gun) and finding a spot that they at least roughly point to, or create a vanishing point on its own layer and trying it out around the horizon line. Then redraw the image, but this time 3 using your new perspective lines. As you paint, leave your perspective on its own layer, on a low Opacity: you’ll have a constant guide to make sure everything’s correct in relation to one another. This will ensure believable foreshortening.