Turn the phone sideways so the camera lens is on the right side and clamp it into the holder straight.
Open the camera in video mode and tap 0.5 above the shutter: that is the ultra-wide lens, and it fits the whole court. No 0.5 on your phone? Use the widest setting it has.
Frame but do not stick - on the recording screen: you need to see the far baseline, the far service line and the near service line. Missing one? Move the holder a little higher or lower on the glass until all three are in.
Wipe the glass clean and dry, then stick the holder in the middle. Press it firmly and give it a tug: it has to hold for the whole match.
A full match is a long video. Make sure nothing cuts it short.