I will do a sketch and refer to another pattern that might have aspects I want to incorporate. Starting out with good measurements is key. Bear in mind that a facsimile done in woven material, or paper, won't 'fit' like the knit. (Read 'Sweater Design in Plain English' by Righetti if you're interested in how a seamstress became sweater designer and how much the two crafts differ.) Once you understand how the stitches stretch, drape, etc you need to incorporate that into your work. Your respective st pattern will effect your results as well. If cabled, sts will draw in horizontally. Certain st patterns will be stiff and have little stretch, others will be very loose. You could do something like linen stitch and get close to how a woven fabric would behave. How much ease are you allowing? Some fibers either shrink or stretch vertically...plan for this. Most likely, as noted, until you've started and frogged a lot of projects, no way to know exactly how something will come out until you actually work on it. How you perceive it in your head, or on paper, may not be what happens with the 'sticks and string.' But that's all part of the fun...and learning process.
I've designed and discovered more than once, as Righetti discusses, finding the marriage between design and fiber is crucial. When the two don't click, it's back to the drawing board. But when it happens....yeah!