It is not that difficult but it does take some math skills
1) mark your two corners.
2) measure the the length of one section of the design of your stencil.
3) measure the width between your two ends from your corner block you marked. Then divide that by the length of one section (step 2) Usually, it is easier to stretch out the design but if it is close, then you squeeze it (usually in the center of the section. I then mark with a pin so I know how to center the section and start making. If the fabric is dark, I use white powder chalk so if you make a mistake, the chalk is easily removed.
Hope this helps.