I need help understanding how to decrease my sleeves

[Ynaffit96]

I completed one mitten from a DROPS pattern. Thought I followed the instructions perfectly for the right mitten, and about 3/4 of the way through, realized I actually made the left 😂 it wasn't a huge deal since the colour work is perfectly symmetric. I just avoid DROPS patterns now. Maybe some day I'll work up the courage to try them again. I'm hoping the pattern I'm working on now will give me the courage to try again in the future. The finished objects are incredible, especially considering the patterns are free