As the name suggests it is worked sideways. You cast on one of the side edges and work your way up, decreasing on every right-side row.
The rows get shorter as you progress. It’s a lot of knitting per row in the beginning, but it gets easier as you go.
You’ll need 4 x 50g balls of Merino Lace yarn in different colours. And you can follow the colour stripes in the pattern or create your own colour combinations.