Hi Michael-
In general, recording, automation, and the mix down process are three separate things entirely. So no, you do not need to set the automation on "write" while recording nor is it something that happens during mix down. Some programs are an exception to this (the program Reason must have the record function on to write automation). Mix down is something that happens when you are 100% satisfied with your song, or just want to cut a demo.
Your missing step is after you are done recording. Set the automation on "write" for whichever specific track you want to adjust the level on. For the programs I know (Pro Tools, Nuendo/Cubase), you can then adjust the levels two different ways. While the song is playing , you can move the fader up/down for the specific track you want to adjust. Then you can stop playback and change the automation to "read" which will tell the program to playback your new or existing automation.
The second way: Look at the automation you just created by clicking on a "+" or "->" sign on that specific track in the project/editing layout. Here you can select Volume amongst other things and you can edit what you just did or create additional automation with the song either stopped or playing. From my experience, all pro software lets you adjust automation either real time or with a series of lines.
I hope this helps, if not just shout at the computer. Buuut... that doesn't always work.