Lay out the t-shirt on a flat surface. Spread out all parts. The back part and the front part should be totally aligned with each other to make cutting accurate. Determine the size of the pillow you want. This could depend on the size of t-shirt and pillow form (filling) you have. Make a paper template of the rectangular area you need. The pillow case should be at least 1 inch wider on all sides than the pillow form that you have.
Translate the template on to the t-shirt using sewing chalk. Position the template in a way that the design you want to keep on the pillow case is included in the right place. Cut out both sides of the t-shirt according to this template. Both sides should be equal. Discard of the remaining parts of the t-shirt. You can also use them as filling for the pillow along with other old fabrics.
Sew the three of four sides of the fabric pieces together, leaving the last side open for filling. Leave a seam of an inch on all sides that you sew from the edges. Sew slowly and carefully because t-shirt fabric is usually very thin and stretchable. However, do not stretch it while sewing because it can distort the measurements and fit.
Turn over the pillow case so that the front side which is the one you want to display is showing. Fill in the pillow form and sew the fourth and last side as well. For this part, you will have to hand sew, using very small and fine stitching to hide it from the obvious eyes. Your pillow is ready to be pounced upon.