Work Flow for a Series of Steps - Split Screen, Chroma Key, Colour Grading, etc

Hello all!
I have recently started creating content using FCPX.
I have taken some video footage (shot on a Canon 77D DSLR)of a guitar player, singer and bassist…. all separately/individually performing
in front of a green screen.
I am hoping to do the following processing –
1. Arranging the main video clips of the guitar player in the timeline to sync to the main audio track
2. Arrange clips of the other musicians on top of the main timeline to create a split screen video [divided into 4 equal parts - 970 x 270
each]
3. Replacing the background (Chroma-key) with an HD image(plain brick wall, 1920 x 1080) for all clips
4. Basic colour correction/colour grading
5. Add .png/.jpg images of guitar chords in the center of the screen
6. Export the whole project to a format up-loadable to YouTube
If y’all could very kindly assist me with the following information –
1. What should be the correct sequence/work flow to accomplish the above steps? Should I first change the background and then do
the split screens, colour grading, etc.?
2. How do I split the screen into 4 equal parts + the center box for the guitar chord?
Any tips/techniques would be most appreciated.
Cheers!
Be well

1. You have the steps in the correct order.
2. Shirnk the 4 clips to somewhere between 50 and 25 percent, arrange them as you'd like. OR get a 4-way split screen template from a developer and just fill in the Drop Zones, but this could interfere with your audio sync. I'd do it manually. For the guitar chords, place those images at the top of the stack in the timeline, simple as that.