I’m a GP, here’s my opinion

Can’t have eaten/drank anything for the last half hour

• in principle could alter your BP but I wouldn’t worry too much unless it’s quite a large meal

Feet flat on the floor

• yes, this is important

Lying down but sitting up

• for some purposes docs want lying/sitting/standing but for home measurements do them sitting

Back against the chair

• yes

Don’t cross your legs/ankles

• yes, feet flat on the floor

Only use your left arm

• myth, if there is a significant difference between your left and right arms there is something funky going on with your subclavian arteries

Hand facing upward/downward

• not super important

Keep your arm down/raised

• keep your arm relaxed, ideally resting on a table or desk at 90deg or hanging straight down

Most important is be relaxed, sit still, don’t move your arm, of you get a high reading calm yourself and take it once more then leave it.