ok so i kinda took it upon me to answer this constanly asked question with a step by step pictorial.
Step 1) You'll need some essential tools. here's what i used; wire cutters/strippers, crimps, screwdrivers, zip ties, sharp razor blade, and some heat shrink tubing.
Step 2) You will need some parts. your choice brake light equipped tail lamp assembly, a factory honda brake switch, a 4x10mm screw (to hold the switch in place under your master cylinder) and a spool of 16 gague wire or lighting equivellant to run for power. i used an 05 yamaha R6 tail light. needles to say this can be done with any 3 wire tail lamp. i then ordered a switch from the dealer part number 35340-ML4-005. this ran me about $15 bux for the switch and found a deal for the tail lamp for $20 bux.
depending on your choice of tail light, you'll need to come up with a method to install it to the rear of the bike. here is a thread on how i did mine should anyone want to do the same style.
Step 3) Next, you'll want to mount your switch with your 4x10mm screw to the bottom of the front brake master cylinder. make sure it lines up with the notch in the casting. you'll want to add some green or light duty locktite to the screw. place the screw into place and snug down. (BE CAREFUL NOT TO OVER TIGHTEN THE SCREW. IT JUST NEEDS TO HOLD IT THERE, NOT SUPPORT THE BIKE!!!)
Step 4) i used some pre-made insulated 2 wire for easier routing through the bike but you can follow the same steps with whatever you have available. take your ends and make a power lead for the battery connection and one for your brake light power lead.
disconnect the power lead from your battery in order to connect your 12V lead to the battery terminal. (i disconnacted the battery holder to slide out the battery to cut the plastic in order to hide the wire. you can skip this f you wanna save time) connect your lead to the terminal and screw the lead back in.
Step 5) connect your brake light lead to the lead off the brake light.
run your wires as cleanly as possible through the same route the wiring harness for the bike runs. using the metal tabs, run the wires through there in order to retain them.
Step 6) you will want to find a way to secure the wire near the steering stem area so the wire has a clean path to hinge from without pulling on the wire when your bars turn. i found a good place right on the brake line bracket. i used some cut fuel tubing to insulate that peice of the wire from vibration against the sharp ends of the zip tie and steel bracket.
Step 7) now, since the wire insulator i was using was grey, i decided to not only further the insulation process (cus im anal that way) but also color match the wire to the rest of the cables running next to it with some black heat shrink tubing. if you are running single individual wires you will definately want to run some sort of insulator to cover the wires from element exposure.
Step 8) you will now crimp on some connections to your leads for your brake switch. when clipping on the leads check your tail light to make sure you didnt cross them. youll want to check em by just squeezing your brake lever and see if your brake light lights up. if it does, it should turn off when you let off the brake. stuff some dialectrical grease in your connection leads and clip em on.
with the leads connected, make sure you have left enough slack so the wire does not try and pull out of the sockets when fully turned to the right. this is where the bars pull away from direction the wires are coming from.
with that, you are all done. you should now have a live feed to your brake light and good to go!