Hi, I'm no expert, although I've been doing quite a few of them lately. A few suggestions are to work fairly wet, put your first colour at the free edge and then lightly graduate it in the direction of the nail plate. You then put your second colour down a little way away from the first and lightly pull it towards the first one, taking care not to obliterate it. The two colours should then start to look as though they're fading into each other as opposed to being two distinct blocks of colour, iykwim? You need to work quite quickly or you won't be able to get the glitters to merge.
Hope that helps a bit until someone more experienced posts.
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