Just to expand upon the answers above a bit . . .
At one time you could simply walk into a pharmacy (or other collection point) and ask for a pack of lateral flow test kits. You now have to get a 'collection code' first, which you can do here:
https://test-for-coronavirus.service.gov.uk/collect-lateral-flow-kits
Once you've got your code, you take it to any collection point. 'Collection points' are often pharmacies but some community centres (etc) are also participating in the scheme. (For example, our local library is a collection point). You can find your nearest collection point(s) here:
https://maps.test-and-trace.nhs.uk/
The kits come in packs of seven and you can collect either one or two packs with a single collection code. The instructions that come with the kits are very clear. They can look a little complicated at first but, once you've done a test or two, you'll realise just how simple they actually are in practice. (I've been taking lateral flow tests, twice a week, for the past eight months and I don't really need to think about doing it by now; I just get on with it!)
There are actually two slightly different types of tests that are being handed out at collection points. (My local pharmacy might give me one type on one occasion but the other type when I next go). Basically though, what each kit contains is a small amount of test fluid which, depending upon the type of test that you're given, is either already in a little plastic tube or in a tiny pouch that you then squeeze into such a container. There's also a thing which is a bit like a cotton bud (which can either be long or short - see below) and the actual test strip that will show you the result of your test.
If you get a kit with a long 'cotton bud' in it, the instructions will tell you to wipe your left tonsil with it four times and then to repeat with your right tonsil. (If you've had your tonsils removed, use the sites where they were instead). Then you push it up one nostril and rotate it around a few times, as specified in the instruction leaflet.
If you get a kit with a short 'cotton bud', you don't have to wipe it against your tonsils. Instead, you push it up one nostril and rotate it around a few times (as per the instructions) and then repeat with the other nostril.
Once your 'cotton bud' has collected a sample, you place the end of it in the tube of liquid and rotate it around for a while, as per the instructions, squeezing the plastic tube when you take the 'cotton bud' out of it.
You then place the attached cap onto the tube. That gives you what looks a bit like a tiny washing-up liquid bottle. You invert the tube and allow a few drops to fall onto the test strip, where it's marked 'S' (='sample).
[With some kits, you use just two drops. With others, it's four].
You then wait for the prescribed time (which is either 15 minutes or 30 minutes, depending upon the type of kit you've been given) and look to see if there are any lines showing on the test strip. You DO want to see one against 'C' (= 'control'), as that shows that you've carried out the test correctly and that it's worked. You DON'T want to see one against 'T', as that indicates a positive result:
https://www.northamptonshire.gov.uk/PublishingImages/LFT.jpg?RenditionID=18
You're then expected to report your result on the NHS website, here:
https://www.gov.uk/report-covid19-result
(Note that, for both ordering tests and for reporting the results, you can choose whether to log in to an NHS account or simply do so on a 'guest' basis. I strongly recommend creating an NHS account, if you've not already got one, as it will save you a lot of typing in future).