You need to get a BRP within 6 months upon arrival, but it's better to do it as soon as possible, as it will make a lot of things much easier. If you submitted your biometric data before entering the UK, then you will have received a visa confirmation letter with an address where you can pick up your BRP. You'll need to pick it up in about a week after your arrival and have your passport with you.
If you did not submit your biometrics to get in the UK, you will need to do the following.
- Fill in the form: https://apply.visas-immigration.service.gov.uk/product/ukraine-scheme-brp . It is very similar to your visa application, the only difference is you'll need to state that you are in the UK. You will need only your passport.
- You will be asked to pick a biometric collection centre closest to you and submit your biometric information. It may say you'll need to pay, but do not worry: the Ukrainian visa scheme provides BRPs free of charge. You can choose to pay for premium services to get it faster, but it's not usually worth it. Choose a center closest you. The centres are very busy, so the nearest date may be a week or more away. Don't be late because you can lose your slot and have to start over.