Pattern directions are very literal.
I find it helpful to write out my directions. Such as in your case:
[COLOR=Red]Round 1-3: even[/COLOR]
[COLOR=Blue]Rounds 4-6: even [3 rounds even]
Round 7: decrease as set forth [4th round of the sequence]
[COLOR=DarkGreen]Rounds 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 37: even
Rounds 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39: do decreases as set forth
[NOTE: Rounds 8-39 fullfill the job of dec every other round 16 times]
[COLOR=Black]Now you mention that something else might need to occur within (at the same time as) Rounds 1-39. Just figure where they should occur and write that requirement in the appropriate round as an extra special chore.
I have been knitting since the Flood, and I still write out my 'job requirements'. It prevents errors, and really, after the writing out is done, the knitting just sails along. You know exactly what you should do, when and where! No guessing, no frogging.
Hope this helps...and I am certain that other knitters may also have something helpful to add!
Knit on Good Woman!