1) Measure your shirt. I measured the bottom width and multiplied by 2. Then I worked out the maximum height for the silhouettes. For me the pre printed text restricted this but with a blank shirt, you are only limited by the shirt size. As I had a rectangular shape to fill (under the text) I just created a new image in Photoshop that was 42" by 10".
2) Find your silhouettes. If you are artistic then you can skip this step. I just did google image searches for silhouettes of things I like. For example to find the Blues Brothers image i searched for "Blues Brothers Silhouette". Obviously most all the images you find will be copyright - I will not be held responsible for any copyright infringements. Stick to personal use of the images and you shouldn't have a problem.
3) Once you've found your silhouettes, copy them into Photoshop. Then resize and arrange them to fit your space. Once you are happy, move on. At this step, you could also add your own designs or modify the silhouettes. I did this with the bike to remove some detail such as spokes and cables.
4) Add a grid. I just found a grid that was the number of squares I wanted then resized it to fit over the image.
5) You might be able to see in the photos that there are a few stray dots. These are where I transferred the points of where the grid lines met up so I could easily transfer the image. Basically, just place a dot for each interesection. In effect, if your grid is 20 lines across then divide your shirt width by 20 and add a dot every however centimetres/inches.
6) Grab your permanent/dry erase marker and start drawing. The point of the grid is so that instead of focusing on the whole image, you just focus on a little bit at a time. This makes it easier to copy. I started on the left and moved right so there was no chance of smudging and I didn't transfer a square to the wrong place
7) Where you design and show off your creativity.