You have to really measure your fender to find a centerpoint that is accurate. Otherwise they will be uneven on both sides. I decided where I wanted to install the sidemarker and then made a latex template from there so that it stayed in place and was even while I made my cut.

So your gonna want to get a dremel for that with a shitload of cut off wheels. And remember to start small and work your way out, better to cut out too little then too much. If your using honda sidemarkers, they are fat on the backside with a hook slot and clip that relies on a good cut to secure it to the fender with no gaps. And the rest? Like Dave said...