How to use a blood pressure monitor?

S • mama to a babygirl 💗 expecting baby #2 in May 💞

I’m almost 35 weeks pregnant and I’m on bed rest for high blood pressure. I’m borrowing this ReliOn BP monitor from my MIL, and I don’t really know where to put the cuff on my arm. I know it goes above my elbow, but how far? I read that it should be an inch above my elbow. I also don’t know what the green line means. Should the green line be in the crease of my elbow?

This is the way I’ve been using it ⬇️ with the wire centered over the middle.

But I’m wondering if this⬇️ is how I SHOULD be using it, with the green line centered in my elbow crease

Does anybody know? I feel like the readings I’ve been getting are inaccurate. For example I took my BP earlier and it was 147/83 and then I took it again immediately after, didn’t change my position or anything, and it was 125/66. The batteries in it are brand new and I usually plug it into the wall anyway.