In nearly all examples, both the raw (-R) and slurp (-s) flags are used:
$ jq -s -R -f [n]-[a/b].jq [n].input
If the command doesn't work, then remove the -R flag and it should.
For example, day 6 might be:
$ jq -s -R -f 6-b.jq 6.input
> 523
Each day has a and b files for each part of the problem. Each day (so far) also has an .input file.
In some cases where I needed to better understand the shape of the problem, I create prototype test files to complement the puzzle, and I tend to use that as the basis for my solution.