Having just done this same installation this past weekend (and also being a DIY noob), here are my comments/suggestions:
- The absolute easiest installation will be if you bought a similar sized microwave from the same manufacturer as the old. This will increase the likelihood the bottom supporting bracket and the hole patterns of the bolts that come down from the top cabinet are the same. This would allow you to simply remove the old and insert the new with little effort since you wouldn't need to monkey with drilling new holes all over the place, worry about leveling, etc.
- Unfortunately my experience tells me that OTR microwaves come in all shapes and sizes and require their own "gear". At one point, I had 3 different makes/models sitting in my kitchen (GE, Kenmore, and LG) and all three had different patterns for the bottom supporting bracket and different overhead bolt patterns. This meant we had to remove the old bracket, mount a new one at the proper height, location, etc.
- Fortunately the new microwave we ended up installing came with paper templates that taped to the back wall and under the overhead cabinet that shows where the new holes need to be drilled. It probably goes without saying, but most OTR microwaves are pretty hefty (40-50 pounds) and at least one of the wood screws that mounts the bottom supporting bracket needs to hit a stud.