To make sure you're safe while DIYing, and to understand where your hormone levels are, it's important to get blood tests. It may be possible to convince your GP to do these if you make clear that you'll be taking hormones either way, but if not, there are also companies that provide tests:
Testing your testosterone and oestrogen levels will help you figure out how much you need to take and whether you also want blockers to reduce your levels of hormones that you're not taking. On the health side, for people taking testosterone it's strongly recommended to get blood tests that include Liver Function Tests and Full Blood Count (of the providers listed, only OptiMale does this). Oestrogen is safer, but Liver Function Tests are still recommended (Full Blood Count isn't relevant to the risks of oestrogen therapy).
If you're using a gel and the blood sample is taken from an area of skin with residual gel on it, it can affect the results. For three days before a test avoid putting gel on your arms if you'll be getting a blood draw (typical if you get it from your doctor), or wear gloves while applying it if you'll be getting a fingerprick test (typical for at-home kits). Wash the area thoroughly beforehand.
Once you're on HRT, try to get your blood tests done as close as you can to 24 hours after you last took it. This is referred to as testing at trough, ie. at the point where your levels are lowest.
It's not absolutely required to get baseline blood tests before starting HRT, but people who don't often regret it when they get later tests and don't know how much their levels have changed since starting. This is a lot of information, so do ask in our support group that we'll link you to if we're providing HRT to you if you need help sorting it all out.