For a rail fence, you cut your fabric in strips or blocks. You may not have 42 in strips from clothing, so you can cut 2 1/2" by 81/2" rectangles. Sew them together in fours (If using strips, sew four together and cut into 8 1/2 blocks.) Press before cutting and square up the block after. Now lay our your blocks turning them as you go. Sew them together in rows, press, then sew the rows together.
Here's quilterscache's - she gives good directions. Note: Hers is three strips, mine was four - You can use 3, 4 or 5. The number used and the width of the strips determines how big you cut the blocks. You want it square, not rectangular.
http://www.quilterscache.com/R/RailFenceBlock.html
I'm doing a Christmas one with five. Scappy just means non matching fabrics as opposed to buying yardage so everything matches. Using clothing, you'll be doing scrappy.
Here are some examples. You won't get that definite diagonal line using scrappy, but that's okay - or you add a solid or something that reads solid. How old are the boys? I am planning mine so I don't have the diagonal line.
https://www.google.com/search?q=rail...w=1280&bih=906