I started with a white base coat, and once dry pressed on some fishtail patterned stencils. Next I watered down some acrylic paint and dabbed it onto the nails one colour at a time, with a small nail art brush. The trick is to keep it nice and liquid so that it pools around the edges a little, this didn't work perfectly on all my nails but I think in general it did the trick!
If the colour was too dark I just dabbed on some more water, and if it became to faint I added more paint. It's a good technique as you can be so rough/vague with it. I left it about a minute to dry a little, but didn't leave it too long as I didn't want to risk peeling off the design. It's important to peel up each corner and carefully work your way into the middle to reduce the risk of peeling. There were a few areas on mine that leaked out a little onto the white area. Some of this I scratched off with an orange stick, but the rest I left on for an edgier look (at least that's my excuse!).
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What I used: White nail polish, acrylic paint (blue, purple and green), fishtail mermaid nail stencils (from eBay), nail art brush and clean up brush, small pot of water, glitter topcoat. silver metal shells