I think this is where things fall apart for you.
Leave the guidance in GPS (Magenta). Don’t touch it at all.
As long as a) you are a reasonable distance from the FAF, and b) you are descending or level at the published crossing altitude to for the fix (i.e., 1800’) then just hit APR.
It will automatically switch to LOC1 (Green Needle) at the right time, if not immediately at that point.