Be sure to stabilize your t-shirt pieces by putting a light-weight knit fusible stabilizer running in the opposite direction of the knit in the t, i.e., t fabric stretching right to left, stabilizer stretching north to south. Cut the t-shirt pieces larger than what is needed, add the fusible according to package directions and then cut the block to the correct size.
If a stabilizer is not used, the t-shirt seams will stretch and you will not be a camper. (Sorry, be a happer quilter!) And I doubt you will be happy with the results.
Suzanne