How good are you at keeping track of your accounts? Anyone selling on Etsy should have some kind of system for recording incomings and outgoings, whether it's a pen and a notebook, or a fancy software like Craftybase (https://craftybase.com/). Do you store your receipts in a shoebox or do you scan them all in to your PC?
Here's some tips from the Etsy seller handbook on more organised tax filing: https://www.etsy.com/uk/seller-handbook/article/4-tips-for-a-more-organised-tax-filing/43167406303. If you want to use spreadsheets to record things but aren't sure how to design one for yourself, then you can purchase one from a fellow Etsy seller like this one: https://www.etsy.com/uk/listing/498097475/etsy-accounting-bookkeeping-software-for
Here's a guide from gov.uk on what records to keep for taxes when you are self employed: https://www.gov.uk/self-employed-records/overview
If you get into the habit of recording things regularly it will make it a lot easier to do your taxes when the time comes. It will also help you to notice the busier months. See here for 5 more tips - to ace your bookeeping: https://www.etsy.com/uk/seller-handbook/article/5-simple-ways-to-ace-bookkeeping/22657834369