As cute as those slouchy suede boots were when you bought them, it's annoying when they won't stay up. While slouchy boots are supposed to have a natural slouch to them, they should not be falling down around your ankles either. You may have purchased the wrong size boot or the dimension don't work well with your body type. Whatever the case, you may still be able to wear your boots the way they are supposed to be worn, with a few simple tricks.
Double-sided tape is typically used to hold a dress in place, sticking to the body on one side and the dress on the other. Insert double-sided tape around the inside rim of the boot, press the boot to you so that the pieces of tape stick to your leg. This is ideal if you are not going to be moving around a lot. Otherwise, with the constant walking and other leg movement the tape will come off of your skin.
You may not be able to wear the boots with just your bare legs. Wearing a pair of jeans may give you the extra padding needed to hold up the boots.
If you prefer to wear your boots with leggings, you can pin the boots to the leggings to hold them up. Using several safety pins, attach them to the inside rim of the boot, just like with the double-sided tape. Once you have the leggings on, put the boots on and pin them to the leggings.
Take the boots to a shoe repair to see if they can take in the boots for you to have them fit better.