Ok, so I don't actually have any experience writing patterns. I've written a few down for my own use, like when I make up a mitten pattern, and have to make sure I can replicate it for the other hand! So I'm going to give you the basic instructions for this shawl I made, and I'm happy to answer any questions.
I made a somewhat small shawl, because that's all the yarn I had of this type. But after trying it on, I've decided that I really like how it's shorter in front - it's handy for keeping your back and upper arms warm, but you still have good mobility. I haven't decided how to fasten it yet - either a shawl pin of some sort, or I might try to work a button fastening.
To start off, make 5 chain stitches and slip stitch them together in a loop.
Make approximately 8 double crochet stitches in this loop (this number will depend on the size of your stitches and the weight of your yarn - you want to make enough to go about two thirds around the circle)
Turn, chain 3, and double crochet twice in each stitch to the end of the row.
Turn, chain 3, and double crochet twice in the first stitch, once in the second, repeating until end.
Turn, chain 3, double crochet twice in the first stitch, once each in the next two, repeating until end.
This process will continue throughout the entire piece. It's how you expand a circular (or semi-circular) piece so that it will grow outwards but still lay flat, without curving like a hat or bowl. For each row increase the number of solitary double crochets between the two double crochets (increase) by one.
Make five rows of double crochet from the beginning.
On the sixth row, double crochet in the first stitch, then chain 1, skip a stitch, and double crochet in the next stitch. This creates the lacy row. When you hit a spot where you need to increase, double crochet, chain 1, but then double crochet in the immediate next stitch rather than skipping one.
Between each 'lace' row, make three rows of solid double crochet. You can continue this pattern as long as you want to make a bigger shawl. I ended up with six 'lace' rows, and added one on the end to make a border.
Making a shawl that is more than just a half circle means that it is slightly shaped, and will stay on your shoulders fairly well even without a fastener. It also reduces the bulk that can build up at the back of your neck with folds from a circular or square shawl.