Self Assessment Checklist.
Make sure you have your gateway account with HMRC and have your login details. (Activation codes can take up to a week or more to arrive.)
Make sure all your receipts are in order.
It’s easier to have a system to update daily/ weekly or Monthly, depending on how many receipts you have.
Know what is an allowable expense:
please see link.
Have your milage to hand, there are some really useful apps that help with this I use MileIQ.
If you’re using the self employed app by QuickBooks, a mileage tracker is available on it make sure it’s activated and kept up to date.