Start with your bike on the center stand.  Use 2 wrenches (I believe I used a 12 and 14) to loosen the lock bolts on the long protruding piece shown all the way to the right bottom corner of this photo.  Do the same to both sides. Next you remove the cotter pin.  It rests through a hole right above the castle nut. It can be bent back into place, so use a pair of pliers to straighten it out and release it.   Use a screwdriver (for the long stem, not the end) to hold straight the bolt on the right (you should see a hole through the middle pointing towards the ground), while loosening the castle bolt on the left.  You can’t miss it, it legitimately looks like a castle.

You don’t need to remove the castle bolts, you just need to loosen them. Once loose, your axel will be free to slide forward and backward.  You will see small notches indicating where to align the axel.  Mine calls for it to be at 3/4″ of slack, so adjust accordingly. The specs will be in your manual.

Make sure that both sides match and are aligned correctly, for the right amount of slack.  Once you have them set, you can carefully tighten the lock bolt, without adjusting the axel further.  Once you’ve tightened, check the slack again on the chain.  Then you can begin tightening the axel bolt (or castle bolt).  You should need a 23mm wrench. Align the hole that you removed the cotter pin from with one of the openings in the castle bolt. Reenter the cotter pin and bend accordingly as it was before.  Once everything is tight and secure, recheck the chain tension, to make sure nothing has moved and you should be good to go, with the proper slack on your chain.