I can follow the 12/8 is to C example but I am unable to wrap my head around 18/16 is to 3/4. Which time to take as the basis, and what tuplet values and pickup bar settings? I need some handholding on this one.
Ok. I suppose the “base” meter is 3/4 and the other one is 18/16?
Start creating your 3/4, in the meter popover, enter. Then invoke the caret in the staff that needs 18/16, invoke the meter popover, write 18/16,12 alt-enter. This should create a pickup bar with 12 sixteenth notes, so it should align with 3/4. Next bar, invoke the meter popover, 3/4, alt-enter. All barlines should align. Select that 3/4 marking and in the properties, hide it.
Then all you need to do is create a 18:12x tuplet in every bar of your 18/16 section (18 16th notes in the time of 12) and tweak its properties so that neither bracket nor number appear. Leave the signposts visible, it should appear as a green 18:12 signpost, from memory. You can alt-click the signposts to make them appear in every bar and only do that once.
Hope it helps!