How do I tile straight in a internal corner?

Hi Can anyone help what is the best way to make an internal wall when tiling look straight . I had my first attempt at tiling a few years ago still not that good and it has survived at the back of the shower and side of shower. My previous attempt I tiled in to the corner and cut the tiles to fit and no edging and a thin line of sealant between the two. This time I was going to try using a plastic strip and whole tiles and have the cut tiles at the end of the window and by the sink to hide not very good cuts . But I am not sure which wall the plastic strip should be on as I have not found a edging strip for an internal corner and the corner is off so would it work anyway . Any advice please . Thank you

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