You will need...
Quite easyTo say this DIY is a total breeze might be tempting fate but I didn't find it especially hard.
Total costMine came to about £6 because I struck it lucky with the top!
Crop it like it's haute...
Turn your crop top inside-out. Fold it in half, across the front and pin all the seams and edges together, as much and as accurately as possible. Sketch out your design using something contrasting and opaque which doesn't rub off easily - I find metallic pens do the trick. Pin all the corners of your designs together - as shown in the picture - and use them as coordinates for tracing a perfectly symmetrical design on the opposite side. Repeat this process with the back.
Fold back all your edges, along the original outlines of your design, and iron them in place with Wonderweb. You can secure them even better by sewing inside the edges but this is optional.