Hi Judyth,
Sounds like you know what you want to do, so go for it!
The first thing you will want to do is a swatch to determine the gauge of your yarn. I'd suggest using the cable pattern you want for the sweater, as this will be the most critical item for constructing you own pattern. Be sure to determine both the stitch and row count for your gauge too, as both width and length are critical when constructing your own pattern.
If you don't have the cable pattern worked out already, here's an easy one that would work well with the mohair:
http://www.berroco.com/185/185_donna.html It's also important to "map out" the sweater, so take the time to determine key measurements besides the width and length of the body. Take the time to measure sleeve length, the top width of the sleeve, length of the body to sleeve opening, width and depth of the neck opening and so forth.
If you're not sure of the completed measurements of the sweater you want to make, use the measurements of a similar sweater your granddaughter wears now. And if you're a visual person (like me), take a few minutes and draw a diagram of the sweater with the measurements included.
If you're having trouble determining details such as number of stitches for ribbing, etc, find a sweater pattern that has the same gauge and sizing as the one you want to make, and use it as reference. You can also use this to determine body measurements.
Also try and keep the design simple. Stick to a basic drop shoulder design with little shaping in the body. The less you have to worry about, the better. And a trick I learned from an experienced knitter who's gauge sometimes changes slightly from the swatch to the sweater (like mine sometimes does)-make the sleeves first, as they are a bit more forgiving, and a slight difference in finished measurements won't make as much of a difference as it would on the body of the sweater.
If you need to adjust the size slightly, consider using a simple stitch like moss (knit 1, purl 1) between the cables. Looks good, and very forgiving. Mohair yarn also looks lovely in lacy patterns, so consider that as well.
I know there's alot to consider here, but I think with your skill level and sense of adventure, you can do this (and do it well). Keep it simple. take the time to work out your calculations carefully, and you'll end up with a truly original sweater your granddaughter will love.
And as always, if need any help, just ask!
Alison
