Mild dish soap and distilled water for the leather. Rub it with a microfibre cloth and dry. Buff in some leather conditioner afterwards. I can’t imagine snot and spit will be too difficult to remove.

Leather? You gotta use leather specific stuff 99/100.

You’ll want a cleaner and some kind of conditioner. I’ve had luck with weiman stuff over the years, but you’ll usually be okay with anything designed for leather, including shoes and jackets; it doesn’t have to be furniture specific. Well, not that I ever ran across.

The problem with stuff like dish soap is that it strips more than you want. It’ll work fine as far as getting rid of schmutz, but it’s used specifically because it binds to oils, and oils are necessary for leather.

Might as well pick up some soft cloths as well. Microfiber stuff is what gets recommended nowadays, but cotton diaper cloth is about as gentle as it gets for delicate surfaces. I’ve made do with cotton flannel, decent jersey fabric and the like, though. The key is that abrasion is not a friend to leather. Not any furniture surface tbh, but leather in particular.

Basically, you take your cleaner of choice and follow directions. Most stuff is either sprayed on or applied to a cloth. You then gently wipe down the surface. Repeat until as clean as you can reasonably get it. It will never be perfect again though. Expect discoloration and changed texture as a permanent result of the condition it’s in now. That’s not necessarily a bad thing, as leather can be a bit drab without signs of wear. Depends on how bad it is.

Let the cleaner do the work, and don’t expect it to be fast. You don’t scrub, you wipe.

Once you get it as clean as it’ll get, apply your conditioner of choice. Now, a lot of cleaners do double duty, but I find that leather surfaces (as opposed to it hanging loose like clothing applications) really need something that goes the extra mile because it’ll take more abuse. I don’t like most of the shoe conditioners for surfaces, though I’ve only been required to use any for a surface once. Did home health for years, so I ended up doing weird shit on slow cases lol.

Based on that pic, don’t expect it to be a quick job. I could be wrong, but that’s an all day job. Likely multiple sessions over time because damn.

While you work, look out for cracks or scratches. That kind of thing is way harder to deal with, and would best be handled by someone with experience repairing leather. But you don’t want to get a ton of anything in those cracks or scratches because the expansion caused can be a major pain in the ass.