H and welcome to KH! You've asked a great question expecially since thate are several ways to answer it.
The stripes are easy enough to do just by changing yarn color. If you knit the scarf flat, that is back and forth as you've probably done before, you'll need to work in a non-curling stitch like garter stitch (all knit rows). You could also do stockintte stitch (alternating rows of all knit across and all purl across) but you'll need to add on a garter stitch border to keep the scarf from curling.
The diamond design can be worked flat in either garter or stockinette pattern but it will look great on one side and not so perfect on the other side. You can find a knitting chart for the diamond online or draw your own on graph paper.
Knitting a tube hides the wrong side but it has it's problems with designs like the diamond although there are work arounds for that too.