Yeah, you are down to the easy part.
A good sponge and a bucket of water as hot as you can bear. Just keep washing until the wall is sqeeky clean, and then wash some more. Change the water often to make sure you have no glue residue and you are all set. Congrats!
If the paste has hardened and you have enough of it still on the wall. Take cheese cloth and soak it in some hot water and paste removal chemical. Put on wall and wait for the glue to soften. Once soft using a plastic scraper remove as much of the adhesive as you can being careful not to cut into the wall. Then as Charles said. Hot water, sponge and elbow grease will do the rest.
Try a solution of vinegar and hot water!
Scrape down walls first, then spray with fabric softener & hot water or vinegar & hot water and sponge off, light sand, then primer. Good luck!
Maybe skin it with joint compound or completely cover it with 1/4 inch dry wall and start fresh. Maybe contact a dry wall expert and ask what they suggest. I've removed paper and painted over it. Both a major pain! I will never paper again. Hope these suggestions help.
I had to use ammonia water and lots of scrubbing with a good scrub brush, the places that I missed crackled when I painted them so beware!!
The trick to removal of wall paper glue or those pesky bits that did not come off the first time is to keep the wall wet long enough to allow the moisture to soften the glue. This is the main reason why its not suggested to leave any bits on the wall and simply patch it using compound. The compound stays wet long enough to soften the paper and glue you were trying to remove. The result is there is a breakdown of the repair as the glue no longer attaches itself to the wall and the patch fails. Using a heavy soap solution that works as a wetting agent and keeping the wall wet enough long enough the remaining glue can and will come off. Using ammonia in high concentrations for long periods of time can become unhealthy from fumes.
Ideally when removing wall paper is not to get ahead of yourself A common condition that often happens when removing the paper. You spend enough time to soften the glue so the paper comes off but you remove to much at a time. This allows the remaining glue that no longer stays wet because of the paper removal to dry. Once dry it is difficult to keep it wet enough, long enough to soften so it can be removed. Slow down, remove on section at a time and while the glue that remains is still wet, scrape it with a wide plastic putty knife and keep spraying it with fresh paste removal chemical until the wall is clean and all adhesive is gone. Then remove the next section. If the walls are drying out lower the heat in the room and keep spraying the walls as you work. Done properly even with several layers and years of paper on the wall, it will come off without leaving any bits and glue on the wall.
I used a product called Krud Kutter to wash the remaining wallpaper glue off my walls. Got it at Home Depot, but likely available many places. It saved my sanity.
I rented a steamer from Home Depot. $30.00 for 24 hours. WORKED great. Super simple, just add hot water to the unit, plug it in and the paper and glue just melted off the wall. Best of Luck with your job.
When I was painting for clients I used a solution of powdered TSP (Tri-sodium phosphate) with a kitchen sponge thingy that was covered with netting. One of the big problems is leftover bits of paper that may still be on the wall. I ALWAYS used oil based primer (Kilz) just using tools I would throw away afterward (cleaning ugh!) If you don't get all the paper off it will roll up into nubs when you paint.
I use hot water with fabric softener and an old hand towel. Works every time. Smells great. The old dried paste will easily scrape off once soaked with the hot water solution. Great question.
Boiling water and white vinegar in a spray bottle. Wipe off with sponge. May need to spray and wipe a few times.