Make sure existing is perfect. Clean with vinegar. Then hang the new wallpaper. I always use glue for pre-pasted wallpaper to insure adhesion.
That said, I would highly recommend removing the existing wallpaper. You will most likely be able to see the lines and probably texture through the new paper over time.
You can make a DIY wallpaper adhesive solvent with 50/50 warm water & fabric softener (I've seen vinegar posted but have no personal experience). You'll need a plastic scraper so you don't damage the wall. If you want to score the wallpaper, be careful not to pierce the wall behind it. A blade to precisely cut the wallpaper at baseboards, trim, and crown molding (unless you are going to remove). Get a container and put all switch plate covers, outlet covers, and screws in there. Soak the wallpaper with the solution and wait about 5 minutes. Then start pulling off the wall. If you use fabric softener, your house will smell like fresh laundry.
Huge mistake, don't do it as you will regret it.
Remove the old wallpaper first. I am a score and steamer from way back.
https://www.lowes.com/projects/paint-stain-and-wallpaper/remove-old-wallpaper/project
I would remove old wallpaper lst. I know it's lots of trouble but new would look better I believe with old off.
Make sure existing is perfect. Clean with vinegar. Then hang the new wallpaper. I always use glue for pre-pasted wallpaper to insure adhesion.
That said, I would highly recommend removing the existing wallpaper. You will most likely be able to see the lines and probably texture through the new paper over time.
You can make a DIY wallpaper adhesive solvent with 50/50 warm water & fabric softener (I've seen vinegar posted but have no personal experience). You'll need a plastic scraper so you don't damage the wall. If you want to score the wallpaper, be careful not to pierce the wall behind it. A blade to precisely cut the wallpaper at baseboards, trim, and crown molding (unless you are going to remove). Get a container and put all switch plate covers, outlet covers, and screws in there. Soak the wallpaper with the solution and wait about 5 minutes. Then start pulling off the wall. If you use fabric softener, your house will smell like fresh laundry.
Either way - good luck