Diagnal knitting stripe

I am going to knit a Christmas tree motif on a sweater. I have the pattern - but I want to add a tinsel stripe (diagnal). Would it be best to darn it in at the end, or would it look ok if I did one stitch on one row, then on the next row do another stitch one further along (and above the first stitch). Would it be ok to use a “fluffy” or spikey kind of wool on this stitch?

I think you could do either for this. My preference would be for duplicate stitch (darning) but you could work the single sts as you go. It’s up to you. If it’s a very fuzzy yarn that you’re using for the tinsel, then incorporating the sts into the work might be easier than duplicate st. Why not try out a swatch and see which you prefer before you start the sweater.

If you are knitting flat you can use intarsia to get the color in it. If its on the round you’d use fair isle which means carrying the tree color all the way around. I’d go for adding it after I think since its probably a small amount.