I used a #11 Xacto Blade on my last 2 decal jobs. I also had purchased a proper cutting matt several years ago when I started scratch building stuff. You want to cut as close as possible to the actual decal image < 1/16". The blades are exceptionally sharp, so you won't have trouble getting a clean cut, no tears or ragged edges.
I would also strongly recommend that you Gloss Coat the areas you're putting new decals if you want the best result then follow the instructions for MicroScale "MicroSol" and "MicroSet". Some folks say you don't need to, but it helps eliminate silvering (that's where the clear decal backing shows and reflects light differently)
AFter everything is completely dry and your pleased with the laydown, no bubbles, edges laying down. You need to top coat with your finish of choice, Either Dull Coat, if you want a flat finish, or semi-gloss or gloss Lacquer