I don't believe it's on Ray's site anymore. You might email him and ask. The
short version is wire a slo blo fuse in line with the magnet, try a 2 amp
first, if it goes try a 2 1/2 amp, then 3. I wouldn't go higher than 3.

As Lloyd says, use the lowest fuse value possible (slow blow of course) for
each magnet that doesn't blow during use.  That will provide maximum
protection for your playfield if a magnet locks on.  Lower fuse values are
possible when you fuse each magnet separately.  Here's the way I remember
it.  Basically, the power to all magnets runs thru one fuse on the bottom of
the playfield.  What you want to do is mount 3 fuse holders, divert the
power from the one fuse holder to the triple fuse holder, and then one wire
from each fuse will supply power to one magnet.  With the playfield in the
up position (looking at the underside of the playfield), I mounted the fuse
holders just to the right of the left playfield support near the front of
the playfield.  I'll send you a small picture I still have from Ray's site
that shows the mounting location.  As you can see, it is an easy mod.
Requires 3 fuse holders, a couple of screws (short enough to not penetrate
the playfield!) for mounting the fuse holders, some slow blow fuses, a bit
of wire, and basic soldering skills.  Everything is available at Radio
Shack.  One side benefit is that if one of your magnets does lock on, that
fuse will blow but the other magnets will remain active until you have time
to fix the problem.