Disassembly of the foot switch is not hard but you will want to do so a bit slowly to get the parts gathered. You might want a cup or a mat to gather the parts.
Turn the pedal over and using a medium Phillips screwdriver, carefully remove the metal back cover.
There is a tiny metal pin in the back of the petal. We do not want to lose this.
Carefully tip the pedal over and catch the small metal pin. Place it in a safe place with the screws.
Hold the pedal together with one hand while putting your screwdriver on the metal hinge pin. Keep pressure on the pedal (it is spring loaded) while using pressure to push the hinge pin out.
If you get the pin partially out, you can use pliers to grab one end and remove it.
Slowly release the pressure between the two halves of the pedal. When you open it up, be sure to save the spring that provides the pedal its action.
So if you have carefully disassembled up to now, this is the collection of parts you have.
You can remove the spring and set it aside with the small parts.
Note the cable restraint - black with 2 screws, holding the cable onto the pedal base in the above picture. Using your screwdriver, carefully remove the two screws and the black plastic bracket holding the wire. Do not remove the switch assembly in the center of the base yet.
Using wire cutters, cut the cable about 1.5 inches (4 centimeters) past the pedal base.
You can set the long cut grey cable aside, this project will not use it, but 3 conductor cable can be useful in another project.
You can carefully grab the grey outer plastic of the short cut cable and slide it off revealing three wires: black, white, and red.
Pull the wires through the hole. You can strip 3/8 inch (3 millimeters) of insulation off the red and white wires. We will not use the black wire, it will be tucked behind the black switch housing when reassembling.
Remove the screws holding the black switch in the center of the pedal base and remove the switch leaving just the grey base. Using a rat tail file or a Dremel tool, carefully enlarge the hole that the wire feeds out of the base to accomodate the Mini-B part of the USB cable. Slowly enlarge the top and a bit of the bottom. The sides do not need much. Try hard to not break through the top of the hole.
Feed the Mini-B side of the cable through the enlarged hole. Place the Trinket 5V onto the cable end to get a feel for how much cable you want inside the housing (it is a tight fit).
When you have the cable in position, replace the black cable restraint with its screws. If the restraint does not "grab" the wire tight enough to hold it well, you can place something else under the restraint like a small piece of the cable cut to 1/4" (6 millimeters).
Alternatively, you can cut the extra piece of outer cable jacket you had in the last picture in half, then split it up each side to open. Wrap the jacket pieces around the USB cable making it fatter. Then secure down the restraint screws.
The USB cable should be held snug now by the restraint. Replace the black switch in the center of the grey pedal base with its two screws.
If you have not done so, strip 3/8" (3 millimeters) of insulation from the red and white wires. Solder the white wire to the GND pin on the Trinket. Solder the red wire to the #0 pin on Trinket.
This will cause the switch to close the connection on Pin #0 to ground when the pedal is stepped on.
You can tuck the black wide under the switch or cut it off.