New blades are in the vinyl weeding tools tub, in a cardboard box.
I'm not sure which one to use (45 degree, 60 degree, or ...) I just picked one and it worked fine.
Clean out the inside of the holder of all the crud that builds up in there over time. (turn it upside down and tap it on the table to empty it of tiny vinyl scraps.)
Put the blade in the holder, so that it's just BARELY seen sticking out of the holder...you can adjust it up and down by screwing the holder clockwise or counter-clockwise.
Screw the silver knob tight - so that it can't move. The black top knob adjusts how exposed the cutter is.
Use page 42 of the manual to adjust it (screw the cutting holder to raise or lower it and then adjust with "pen force" to make the circle around the square cut out perfectly.