The ground is part of the shot
A flat range hides a variable that the field exposes constantly: the slope and shape of the ground you're shooting from. A position that's automatic on level grass falls apart on a 20-degree side-hill. The good news is that the principles don't change — bone support, contact, NPA, straight-line recoil — you just have to rebuild them to fit terrain that isn't cooperating.
(Note: the ballistic effect of shooting up or down at a target — slant range vs. line-of-sight range — is its own topic covered in the high-angle lesson of the Sniper Skillset. This lesson is about the positional problem of unstable ground.)
Side slopes and rifle cant
The most insidious terrain problem is the side slope, because it quietly induces cant — the rifle tipping left or right off vertical. Cant moves your point of impact: as you dial elevation, a canted rifle throws the bullet to the low side, and the error grows with distance and the amount of elevation dialed. A few degrees of cant is invisible at 100 yards and a clean miss at 600.
On a side slope your body naturally tips with the hill and brings the rifle with it. Two defenses: level the rifle, not your body — use the scope's level bubble or your reticle against a true vertical (a tree, a post, the horizon) and trust the bubble over your inner ear, which lies on a slope. And build the position to the rifle — dig in your downhill elbow/knee, shorten the downhill side of your support, and let your body conform to the hill while the rifle stays plumb. On a tripod, level the head with individual legs. Check the bubble before every shot on uneven ground; cant is the error you won't feel.
Forward and reverse slopes
Shooting downhill over a forward slope (the ground falling away in front of you) is often the most stable terrain there is for prone — the slope drops away from your muzzle so you can get low and still see down range. Get prone, build normally, and enjoy it.
Shooting uphill or over a reverse slope/crest is harder: the ground rises in front of you and blocks a low prone, forcing you higher (kneeling, sitting, or supported standing) just to see the target. Use whatever's on the crest — a rock, a log, your pack — as support, and accept that you may have to trade some stability for the height needed to see over. This is exactly where barricade and tripod skills earn their keep.
Obstacles, vegetation, and improvising
Tall grass and brush will swallow a prone muzzle — you'll be aiming through vegetation you can't see past, and a stalk of grass at the muzzle can deflect a shot. Get just high enough to clear it, no higher. A pack becomes a bench; a forked branch becomes a rest; a fence rail becomes a barricade. The field-craft mindset is to look at terrain and clutter as a kit of supports rather than obstacles. Whatever you brace against, apply the same rules: soft or braced contact for the rifle, bone support for the body, level the rifle, check NPA, load the system.