You should be familiar with the basic setup of the M4 Eyes project based on the most excellent Adafruit MONSTER M4SK guide Quickstart. Before you proceed with making your own eye pattern texture maps, be sure you've got the M4Eyes.UF2 working properly using the default Hazel eyes graphics.
As terrifying as this may seem, the first thing we're going to do is BREAK the MONSTER M4SK! It's going to be OK, though -- It was designed to be broken! That's what those little perforations on either side of the bridge are there for. We promise!
Cut the Bridge
Using diagonal cutters, snip the perforation starting at one end, then flip the board around and finish the cut.
Then, trim the bridge from the other side as well, using the same procedure.
Plug it In
Next, use the 9-pin JST-SH cable to reconnect the boards.
Note, while each end of the cable is identical, the plugs have polarity and can only be plugged in one way to the board connectors. Don't force them in if they aren't plugging in easily, just flip them around and try again.
Try powering the M4SK and turning it on now and you should see it running just as normal! Here, I've got the default test code running that ships on the M4SK.
Using double-stick foam tape, secure the LiPoly battery to the back of the MONSTER M4SK's right eye board and plug it in.
Depending on your plans for the M4SK, you can now add lenses and lens holders, put on some eyeball .UF2 code, maybe customize the eye graphics as shown here.
For the fish head mask, we'll go about it a bit differently, so no need for the lens holders.