At this point the gs border is set and you will start incorporating the new stitches into the center. If you want more than 4 border sts, just continue in the above manner until you have the number you want. You now have to think about the pattern for the center triangle. You can wait until you have enough sts for an entire repeat before starting the pattern, or find the center of your pattern to place over the center stitch. If your pattern takes an even number of sts, inc in the center st on the next row and these 2 sts will form the center.
Instructions here are for gs borders and ss center, pattern suggestions are at the end.
Knit 4-6 rows of garter stitch and bind off loosely. Diamond patterns are very effective, as long as the diamonds spread at the same rate as the increases. Otherwise, you'll be starting at a different position in the pattern on each repeat. Entire shawl can be made in seed st. or garter st. It doesn't matter which is the center st.