The font size is 12 pt, tracking (aka line spacing) is 18 pt, and 18 pt between guides:
I want the lines to be aligned at the bottom of their line spacing instead of their top. What is the proper way to do this?
Do I need to shift the baseline (that is, to open the basic paragrah settings, select "Advanced Character Formats", and then adjust "Baseline Shift" to -6 pt) or there is a better way?
First of all, you don't need to manually add guides for this. InDesign has a built-in feature for this called Baseline Grid. Here is a quick intro to it.
If you enter Edit > Preferences > Grid you can set up the baseline grid as you like. In your case something like this:
Turn on the baseline grid using View > Grids & Guides > Show Baseline Grid.
Now you have the grid set up, but the text won't automatically align to it:
The simple way of forcing text to align to the grid is simply entering its Paragraph Style Options and set Indents and Spacing > Align to Grid to All Lines:
Now any line of texct using this paragraph style will be forced to align to the baseline grid:
In your specific case there is also a simpler way to control where the first line of a text frame begins.
Enter the text frame's Text Frame Options and set Baseline Options > First Baseline > Offset to Leading.
This way, the first baseline of the text frame will be 18 pt. below the top of the text frame.
Read more about this issue in this answer.
