Tax on Upwork income

Hello,

I have moved to the UK in January this year and would like to continue doing freelancing through UpWork, which I have done in addition to my full-time job in Poland before.

I haven't been self employed in Poland, but at the moment I am starting a part-time job here, so I will devote more time to freelancing (I hope for it to make up around half of my income) and I want to keep everything by the book, so I want to register as a sole trader.

I do have a few questions about how the income should be calculated in the UK, since it is invoiced and paid in USD. Does the rule of using the exchange rate from the day preceeding each payment also apply? And do I use the dates from the UpWork invoices or the dates on which I actually withdrew the money to my PayPal or dates on which I transfered the money to my British account in GBP?

Also, do I report the amount paid after upwork fees, or the gross amount and then use the UpWork fees as deductible expenses?

I know I still have time to submit my first self-assessment, and as I only have partial tax residency in the previous tax year, and I haven't been making any signigicant money before April, I am not going to pay any tax or contributions, but I want to use that low-risk period as practice for the future, when stakes might be (somewhat) higher and I might be more busy.

I would be really grateful for some clarification and guidance.