Step 1: YARN
Choosing which yarn to use depends on a few factors.
What season am I designing this pattern for? Spring/Summer garments tend to use finer yarns DK, Sport, Fingering, Lace and may have some cotton/linen content. Fall/Winter designs will use heavier weight yarns like Worsted/Aran and also can include DK and sport weight. Wool, alpaca, cashmere, angora, and mohair yarns can all be used in the winter. Sometimes the publisher/magazine has a specific yarn they need used. In this case there really is no decision to make.
Yarn choice can also depend on the cable/lace/texture pattern I plan to use on the garment. If I'm doing cables I want a smooth yarn which will show the cables nicely, probably wool or wool/blend. For lace, I like to use finer yarns and nothing too fuzzy.
What about colour? If I'm designing for a magazine/publisher, they often have "mood boards" with lots of things to show colours they would like to see. For instance, this picture below shows a mood board for Harrisville Designs.
To me, this board says "cool", "wintery" colours. I would choose a cream/natural, grey, or maybe an ice-blue colour. Since this mood board is for a fall/winter garment I would use a wool DK/Worsted/Aran yarn.
With all these choices it can be hard to choose a yarn for a design.
Next week I will talk about stitch patterns ... sometimes I choose the stitch pattern first if I can't decide on a yarn.