Lupo, there are a number of ways you can do this. Use a template like the cut-off from the end of your fender or mock it up with painter's tape (that pale green stuff) or use a black marker and free hand draw it, etc. Just make sure it's even and follows a line you like. The cut could be at a right angle to the radius of the fender or more acute and follow the horizontal line of the bike's seat and tank. It's what you want it to be.
I would get rid of the existing taillight and position the new one on the fender. It looks like the taillight you got will fit the stock holes. Mount your number plate on the light. Eyeball the bike from the side and figure out where you want your cuts to go. Is it going to be a straight cut under the plate following through around the sides? Do you want the cut under the plate and let the plate protrude past the end of the fender cut? Are the leading edges going to round up into the radius of the wheel arch? Your bike, you call it.
Once you have found the line that YOU like, mark it out with tape or however you want. You should make 2 cuts. One to get rid of the 'meat' you don't want. That is, cut away leaving only about 1/4" to 1/2" of the fender from your finished mark/tape. This smaller left over bit will make it a lot easier to make the final cut because you can pull the tiny strip away as you cut. Also, you won't have the heavy cut off piece possibly bind up your blade as you cut. Your first cut is also a chance to get a feel for using the grinder. Call it a practice run.
Start cutting on one side till you get to the top of the fender and pull the grinder out of the cut. Move to the other side and cut there to the top of the fender. Either pull the grinder out or continue through to the other cut on the opposite side. Don't forget you can reposition the grinder's handle to either side so you can work with the grinder from either the left or right side. Once the first cut is done, you can then more easily make your second, final cut to get your new edge.
While doing your final cut, leave just a hair of metal from your finished mark to do your finishing. That is leave less than 1/16" or one millimeter from your taped or marked edge. This will allow you to fine tune the finished cut using a bastard file, then finer flat and round files, then 120 grit, then 280 grit sandpaper and finish up using Scotch Brite pads. Be careful not to mar your paint with the sandpaper or Scotch Brite. Wrap them around a small wood block to make the sandpaper work better.
I would use an angle grinder with a Zip Cut disk to do these cuts. You could use a hacksaw but that will be a lot of work. Don't use a jig saw. Just not recommended at all. If you don't have an angle grinder, just rent one that will use Zip Cut disks and get 2 of the disks too.
DO NOT use an angle grinder without leather gloves, face shield (just glasses aren't good enough) and don't wear flip flops. Keep your legs away from the grinder as it's cutting. I have seen what a grinder-gone-wild can do. A buddy wound up in hospital with blood poisoning after a Zip cutter went into his leg about an inch. Wear ear plugs or muffs too. Always keep a firm grip on the grinder. If you let it do it's thing and not control it's movements, well, you got grinder rodeo. That's dangerous.
NEVER start or stop a 'circular' saw / cutter in the middle of a cut. Always full RPM when going into or pulling out of a cut.
Don't have people standing around just watching. Unless of course they at least have some eye and hearing protection. These tools are not meant to be fucked around with. You can't get an eye back.
Make sure the piece you are cutting is stable. If the fender is going to tend to vibrate or wobble, stabilize it by wedging wood blocks between the fender and the tire as close to the cut as possible. It doesn't have to be super tight, just enough to stop the vibrations.
Keep your cuts straight. These things don't like being torqued sideways.
This is really not as bad as it sounds. These are steps, precautions that an angle grinder demands.
By all means, take your time. This is way easier than it sounds and when it's finished... oh ya!
Cheers man!
http://www.worksafenb.ca/docs/HaHandGrinders_e.pdf
This pic, the guy did a good job. I just don't like the line of the end of the fender. I would rather have a horizontal line (to the line of the side of the bike) and the fender would have ended behind the plate. That just looks awesome - nice and creative use of rubber solder on that taillight.
Ummm... nope! Don't do this. Looks like a single cut all the way, not good. Flip flops and a bouncy jig-saw.
