I got the 1100mah pack from Onlybatterypacks.com.
Step 1 - open up stock battery box and snip out the old battery holder by cutting the black and red wires that go to the plastic battery holder (where your 4 *now inferior* alkalines are.....I snip the wires in the center, that way no swearing about making one side too long or short

).
Step 2 - strip 1/4" off of each of the wires remaining in the stock battery box (they come off the on/off switch).
Step 3 - solder red lead from on/off switch to positive lead of battery pack.
Step 4 - solder black lead from on/off switch to negative lead of battery pack.
Optional - use heat shrink to cover both of these connections........I did, but since the connections were so far apart it would be really odd to have them touch.
Step 5 - route wires inside battery box and replace back onto Tmaxx.
Step 6 - let it charge..........charging instructions should be included with the pack (in my case, 11 hours of slow charging for the 1100mah pack).
Sorry I didnt take pics, I hope the explanation helps.
-Chris