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I'd google it, cause there must be some tips.
I did it back in the 90s (blue and yellow). It was alright for a teenagers bedroom. I used my mum, a big spirit level and masking tape. Tbh pulling the tape off badly was what made it a less good job, so I'd try some practice runs of that.
My gut says buy decent tape and a Lazer spirit level. You should be able to position it far enough back to give a long line.
anyone painted a bedroom two tone? other than tape and eyeballing it, how else could I ensure a good horizontal line? First time painting a room lol. Cheers