I am following the instructions closely, and the first step it recommends that you do, is get a sheet of sandpaper and lay that on a flat surface. I used masking tape to tape a sheet of 220-grid paper to a sheet of glass. I then used that to sand down the bottom of the main body piece. If you look closely at the bottom of the main body, you will see a tiny lip. It looks like it is part of the body, but it is not. That is extra material that needs to be sanded off.
The extra bit of material on the car's floor is easier to see, but it requires the same work along its long edges. When you insert the part, look for any tight-fitting areas, as there might still be some extra material that needs to be removed.
I followed the instructions on how to sand the fishbelly frame pieces. This is because there is a layer of material that needs to be sanded off from the parts themselves, that it is equal in thickness to the flashing that you see between the two parts. It takes a bit of time, and you want to make sure to apply even pressure all over the parts as you sand them down. Once the flashing broke, I still had to sand one of the parts down a bit more before it would fit snugly in its spot in the underframe. This photos shows the parts positioned, but not yet glued down.
Next, I hollowed out the elongated holes in the fishbelly frame pieces. In the foreground you can see where I had not yet fully sanded down the bottom edge of the main body, as the casting lip is still visible. You can also run your fingernail across the body's edge to see where it is still present.
The instructions state to tackle the roping loops at the ends of the bolsters, next. There are two small dimples cast into them. I drilled those out, and then continued to drill out a couple of extra holes in between those two. Then, using a #11 blade, I cut and scraped out the remaining bits. The results shown in the photo look a bit messy, but this photo is an extreme close-up.
The brake cylinder part is easy enough to prepare and to install. I used gel superglue to attach it to its two spots.
The brake levers are installed next into the two holes in the fishbelly center beams. By the way, this photo shows damage on the left center beam (near the bottom of the photo). That's how it arrived, but that should be fixable.
Installing the wires that represent the brake lines was a bit tricky to do. The parts are small and the space it tight. I am not 100% happy with the line coming off of the cylinder (to its left), but since that is an underbody detail, I'm going to let it go. It looks OK from the side. The photo does not show the actuator rod with clevis on the right-hand side of the cylinder, but that is installed. Also note that I fixed the broken-off section of the center beam casting. I did that by making the "damage" bigger and rectangular, and then cutting a piece of styrene of the same thickness. I then filed the styrene smooth and even when the glue had dried. Once painted, it should be invisible.