Put the almonds in your food processor (using the S blade).
Turn it on and set a timer for 15 minutes.
After about 10 minutes it will start to clump together. If it starts rising to the top and sticking to the sides, turn the processor off and use a plastic spatula and scrape the sides down. And then turn it back on.
Leave to mix for another 5 minutes (sometimes it needs a few more minutes) and it will turn into a butter texture.