A couple of examples where you can place your designs on different textiles and objects.
Place the design 19 to 22 cm down from the left edge of the collar and 10 to 15 cm to the right from the middle of the shirt. Find the middle by folding the shirt in half. You can iron the folded shirt to get a better crease.
Place your design 8 to 10 cm to the right of the middle/buttons and 15 to 20 cm down from the middle of the shoulder seam. Iron creases to make them more visible.
If you want to place your design on the back, fold the blouse in half and place it 15 to 22 cm down from the collar.