For the T-shirt I made for the boyfriend I bought a simple T-shirt at H&M. I made a Swiss cross stencil out of cardboard and made the pattern on the T-shirt by tracing the outer lines of the stencil with dark grey fabric paint and a small paint brush. After a night of drying I got my sewing machine ready and traced the cross in the lower right corner with a zig-zag stitch using the smallest possible stitch length. I sewed one of my "Catcat" labels on and that's it!
I tried to keep the design rather simple because the boyfriend and I tend to have different opinions about what a great T-shirt design should look like. But I like how it came out. And the boyfriend likes it too!
Sorry for the bad quality of the pictures by the way, I had to take them by night so they're all with flash.