Hi Knitgirl,
I'm sorry to say the answer is that it's not easy. You have to work out each row quite carefully to keep the lace pattern looking correct.
I'm going to confess to not being a lace knitter myself but I've done some research and most of the time I think you work the first and last four sts of the pattern (where you're decreasing) as though they were plain stocking stitch stitches and then continue the pattern as normal. In sizes 8, 10, 12 and 14 the decrease section covers a yfwd and it's respective decrease so you then just continue the lace pattern as written. However for size 18 the first four stitches include only half a yfwd/dec pair so here treat the first five sts as stocking st.
In the following row you will have distorted the pattern so if you don't have enough sts to keep the first part of the lace pattern correct, work those stitches as stocking stitch stitches, then join in with the pattern again.
Here is the article I used to work things out. It may help for you to take a look at it while you work.
It's quite confusing, hopefully you can make something out of my response and the article. Let me know if it still doesn't make sense.
Good luck with it!
Adrienne x
