This script provides a simple solution to create video walls. My solution, I believe, is relevant if you want to create a large amount of screens – like several hundreds. Otherwise it's probably better to go for this one, that I also use myself. (This one and this one are also interesting solutions.)
Here is the workflow:
1 - In Premiere, create an edit of your raw footage. You could also just do precomp in After Effects, if your grid is not too big. This edit should be long. The longer it is, the less repetition in the grid you are going to get. Note that the ratio that you choose for this, will be the ratio of your screens.
2 - Export your edit in low res. The more screens you have, the lower you should go, and the less render time you gonna need. For example, if you create a grid with 10 screens, and your composition is 1920 pixels large, final width of each screen is going to be ~192 pixels. So you can just export like… in 360p or something - slightly larger so it keeps some sharpness.
3 - Bring your edit to After Effects, put it in a comp, select the layer, run the script! What it does is duplicating your original layer, re-scale/re-position it, and randomly time-offset it, to catch different parts of your edit. You'll then get a 20 seconds sequence with your grid.
You can also:
have a randomized fade in intro (ON by default)
get a drawn grid between the screens, in a Shape Layer (OFF by default)
Here is a quick demo (a full tutorial will come one day):