Waylink English is a nice little British site; I think I’ve linked to it before (if not, I should have). It’s nothing special - no blinky lights or guarantees of A++ if you read their articles - but it’s good for people who need a refresher course on formal writing. Browse through the links on the left-hand side of the page and see if there’s anything that might help you.
If it all looks familiar, I suggest you then have a look at this page for practicing formal writing. There are two exercises: a paragraph in which the verbs are informal, and twelve sentences in which you will find twelve “formal writing no-nos”. It’s nothing heavy, just something to keep the brain in writing mode during the summer months.