When you're setting up Folk for the first time run:

# at the root level of the folk project directory
./folk.tcl setup camera

This will allow you to see how much of the table your webcam can see:

When you have four AprilTags printed out, in view of the camera, run:

./folk.tcl calibrate

This will run through gray code camera calibration. A successful run of this process looks like the following two GIFs (note the red triangles on each AprilTag at the end of the calibration process):

If auto calibration fails see manual calibration.