How can I mark placement?

Printed templates are the best tool for perfect embroidery placement. This video shows how to effectively use them.

Print your design template (be sure to enable crosshair printing) on paper or vellum
Cut it out.
Use your printed template to help you decide where to place the embroidery, by holding the template over your fabric.
Use a ruler and a removable marker to transfer the crosshairs from the template to your fabric
Now you have a “target” for your embroidery placement.

More on the various ways of marking placement for your embroidery.