https://www.reddit.com/r/CreateMod/comments/17svwpz/using_signs_to_print_patterns/
The main concept is pretty simple. Just have a sign placed at each block before your pattern starts. The placements of signs can be a little wonky and will fail. Here are some things I figured out.
1. Make sure each sign deployer is one block above where the cart is. (The signs are placed in the wrong order if they are below and therefore don't stack properly).
2. There needs to be a block placed beneath where the signs are going to be placed.
3. The Plough that breaks the bottom sign needs to be one block before the end of your pattern otherwise there will be a gap between each pattern.
4. You can use a string in the pattern to act as 'empty' spots so that the wrong block isn't printed where you don't want any block.

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