Just like so many other sections, the start of section 18 has you prepping, cutting down to size, or locating holes in parts for later use in the section.
Measure twice (or 15 times) cut once really applies here. For the life of me I couldn't figure out to how make the threaded flange fit the drawing's specs. Then the light bulb went off and realized my measuring skills were not up to par during this session. One thing I've had to learn during this project is: If something doesn't go together right or just seems "off," stop and walk away (or go to VAF) and come back later.