You'll need to look at the wall and test the paint. If it's smooth drywall with semigloss paint that doesn't have "stain-free" additives, you can use ordinary cast vinyl like Avery 950 or 3M 7125. Cut some letters and stick them up. If they are still on the wall a day or two later you are good to go.
If the wall is textured or has been painted with the aforementioned type of paint, your only recourse will be to try a hi tack vinyl. We use Arlon DPF8000 for this. It's a digital media so you'll have to print and then cut. Since you'll be cutting through the ink to make this type of letter, give it plenty of time to dry and outgas before cutting. At least 24 hours. Sometimes even the hi tack vinyl won't stick to the stain-free paint. The only recourse then is to repaint the wall. Or do dimensional letters cut from acrylic or expanded PVC and attached with stud mounts.
If you have to use hi tack vinyl there exists the possibility that removing the vinyl will take paint and drywall
paper with it.