A quick web search came up with the following variants on the recipe:
* reboot the phone
* when the phone starts to reboot, press and hold power and volume down together
* when the device logo ("Redmi Note 7") appears release power but continue to press volume down
* look for "safe mode" in the bottom corner when it has rebooted. If not, try again.
OR
* reboot the phone* when the device logo ("Redmi Note 7") appears press and hold the "volume down" key
* look for "safe mode" in the bottom corner when it has rebooted. If not, try again.
It may make no difference, in which case holding volume down all along will be less of a fiddle, but I'd try both.
I did also find a post that said that Xiaomi had removed safe mode in Miui 10 (which if true would be incredibly stupid and a good reason never to buy one of their phones), but that last recipe was posted in the same thread as that statement but some months later, and did stress waiting for the full logo "Redmi Note 7" to appear rather than just "Redmi". So it's possible that the timing is critical and the person who thought it was removed just wasn't aware of that (most recipes just say "when the logo appears"). I've therefore added that stipulation to both versions, but it may be that you have to experiment with the timing so don't give up if it fails first time.
I'm afraid I don't have a Xiaomi device so can't test any of this myself.