To set up a recurring buy, go to the Crypto section in your 'Home' tab or the 'Wealth' section of the Hub, and select 'Recurring buys'. Tap on the + sign in the top right corner. Choose the cryptocurrency, set the amount and how frequently you want to buy it.
You can either start it straight away, or choose a future start date. Once you've read the risk disclosure and confirmed your recurring buy, your recurring buy will begin.