I'm hardly a polished pro, but i'm glad to see the first suggestion: proceed slowly and carefully. I'm going about writing my first screenplay and frankly doing a LOT more reading and studying of others' works than much writing - in large part because I know that I'd rather get it pretty darned close to exact the first time thru and just polish and tweek on my rewrites and edits, rather throw it all out there in a mad race and have to completely redo major sections. I got the sense others work more in the mad dash pace and my caution isn't shared by others. Glad to read otherwise.