This is part 6 of the Using an electronic drum kit with Rock Band FAQ.
First, cut yourself about 5 inches of CAT-5 cable, with both ends exposed. Remove the outer sheath and unwind all of the twisted pairs in the cable so that you have eight separate wires. Strip about one cm from the ends of each wire.
The keystone jack you purchased from the shopping list will serve as the RJ-45 interface for the MSA-P to the Rock Band drum controller. The jack itself as well as the instructions that come with the kit will show you which wire colors should be punched into each respective slot on the jack. Remember we are using the TIA/EIA-568-B standard. Follow this mapping precisely and you should have the correct pinouts. Watch this video for an example of how each wire is punched into the jack.
Note that the jack comes with a simple plastic punch tool so you don't need to get your own fancy puncher.
Connect wires from the RJ-45 keystone jack to the MSA-P
After following the above instructions, you should have an RJ-45 jack with 8 wires coming off of it. You now need to connect those wires to the MSA-P.
The following diagram shows you where to wire everything, presuming you have an XBox 360 (instructions for a PS3 are slightly different and I don't have time to get into it right now). Ignore the yellow wires on the right side of the picture…these are for the MIDI jack and we'll cover that later. The wires going to the terminal block set on the left side of the picture are color coded according to the drum they will activate, so use the above mapping of wire to drum color to figure out which wires to connect.
The only tricky part here is that ground is needed in a lot of places. You can just route wires in series from one ground to the next to connect them all together.
Also note that we are bypassing the voltage regulator of the MSA-P. The MSA-P accepts 7.5V to 12V on its power input leads, but the voltage regulator accepts this and generates 5V regulated power for the rest of the MSA-P.
Fortunately, 5V regulated power is already provided by the USB drum controller. So all we need to do wire the 5V and the ground from the drum controller to the 5V output and ground of the voltage regulator on the MSA-P. As shown in the above diagram, we can actually accomplish this by wiring 5V and ground into the power outputs of the MSA-P. The power outputs of the MSA-P are directly connected to the 5V and ground outputs of the voltage regulator, so making our connections there is completely equivalent and has been verified to work.
You could do what I did and solder wires directly to the pin outs of the voltage regulator, but I can't think of any reason you would want to bother with that approach now that the approach of powering the MSA-P through the power outputs is confirmed as working. See the below picture for the obsolete approach.
Connect MIDI In to the MSA-P
The following diagram illustrates how to connect the MIDI In port to the MSA-P. Picture borrowed from Flash's article on this modification.
Ignore the other portions of the diagram…just worry about the two wires that run from the MIDI In port to the MSA-P.
You will probably want to solder the two wires from the MSA-P to the MIDI In port. This is a really easy soldering job. Just thread the wire leads through the connectors of the MIDI In port, wrap it around once, and apply the solder.
Enclosing the MSA-P
Believe it or not, this is all the connecting you need to do. The next step should be to put the MSA-P in an enclosure so that you have a nice and neat little box to interface with instead of an exposed board and connectors hanging off of it.
Unfortunately, I do not have any advice for you right now on how to put this together. I have a custom enclosure that was made by a kind forum member and those are gone for good.