wire all of the grounds together (the bits that touch the long (sleeve) part of the 1/4" leads), then connect the others (tips) to a DPDT footswitch (double pole, double throw) - the connectors look like this (crudely illustrated)
______
| 1 2 |
| 3 4 |
| 5 6 |
----------
Input goes to lug 4, output goes to lug 3. wire lugs 5 and 6 together, then wire pin 2 to the SEND socket, and lug 1 the the RETURN socket. Ultimately, as long as the INPUT and SEND are on the same side, and the RETURN and OUTPUT are on the opposite side it doesn't matter which way around you do it, just as long as you mount the footswitch vertically.
good quality footswitches and jack are available from loads of different suppliers (you can also get them off eBay, but it's a bit hit and miss on quality on there at the moment)
Reputable suppliers are fuzz dog's pedal parts
http://shop.pedalparts.co.uk
Bitsbox
http://www.bitsbox.co.uk/?main_page=index&cPath=116_128
for example.
The toughest bit about building a box like this is drilling the enclosure (assuming you're going for a metal one).
Don't forget to tin your wires!