You can either paint or pipe the design.
In fact, traditional mehndi is applied with a cone - very similar to piping.
So, just about any YouTube video can be of help.
If you don't want to pipe, hand paint with either airbrush colors, thinned gel colors or colored cocoa butter.
Mehndi are traditionally repeating patterns, so creating a seamless pattern is easy because there is no right or wrong way.
For cupcakes, consider hand painting on pre-cut circle edible printer sheets. Paint 'n apply....
A good design source is Dover Publishing's Mehndi Design. There is a choice of either a basic book or a book with companion CD.
Another source for designs is Stock Photography sources such as ThinkStock:
http://www.thinkstockphotos.com/search/#mehndi/c%3D431%2C253%2C632%2C28%2C34%2C260%2C263%2C13%2C176%2C621%2C648%2C579%2C528%2C590%2C151%2C268%2C515%2C64%2C663%2C641%2C165%2C477%2C623%2C215%2C445%2C637%2C144%2C2%2C452%2C451%2C109%2C277%2C161%2C588%2C626%2C68%2C591%2C460%2C291%2C344%2C629%2C614%2C647/f%3DIEHV
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