I, too, enjoy designing my own knitwear. I have to say, it helps to be really good at math.
Typically, I start with measurements, (usually my own, but often, a grandchild.) Add a certain amount of ease, depending on the intended garment's fit - you don't want your garment to be vacuum sealed to your body. That gives you a finished measurement for the garment.
Now, you must knit a swatch with your intended yarn using your intended needles
. I usually start with about 20 stitches, and 4-6 rows - check how it's going so far, and if I like it, keep knitting to about 20 rows. I frog and reknit until I can come up with a 4"X4" square. This is also a good oportunity to see if you like the needle/yarn combo. Play with needles sizes until you get a nice looking stitch (or stitch pattern.) From the swatch, I can then calculate how many stitches to an inch I am getting, both vertically and horizontally.
If I need, for example, 4 stitches to an inch, and I am knitting a sweater that is 40 inches finished measure, I will have to cast on 160 stitches. If you are knitting in the round, or in one piece up to the armholes, you can start knitting right away. If you are knitting 2 fronts and a back, you'll have to divided the stitches among the pieces involved. (You will be eventually anyway, because you'll hit the arm holes.) So that would be 40 stitches for each front and 80 stitches across the back for a total of 160 stitches.
Of course there are increases and decreases involved depending upon the shaping of the garment. For a beginner, it's obviously best to start with boxy, square, loose garments, learn the basic math involved, and then "grow into" more shapely garments as you practice.
I hope that's a good starting place for your knitwear designing ambitions.