I started by switching an easy feed to formula, so not when they're exhausted. She took to it really well, it turned out a lot easier than i expected. stick to one switched feed for a few days to make sure the formula agrees with her, if not try a new brand, but make sure you give each one a fair try of over a week, otherwise you risk upsetting your little ones tummy more. If all goes well, switch more feeds until they're all formula.
Hope this helps.