Fun Fleece Animal Hats pattern, available as a downloadable PDF pattern from tiedyediva.etsy.com. The antlers are kind of floppy, but that's part of their charm. This works with both Volume 1 (plain style) and Volume 2 ('bomber'/chinstrap style). You can also add reindeer ears if you like; use the cow ear templates from Volume 1. I chose to keep it simple, mostly because I sew during nap time these days, and, well, nap time doesn't always allow for ears and antlers both.
First, download the Antler Template. Click File in the upper left, then Download to save it. Cut 4 antlers, being sure that 2 of them are reversed. Lay each pair right sides together and sew all around using a 1/4" seam allowance, leaving the bottom opening for turning. You'll definitely need your favorite pointy turning tool to help you turn the antlers. I use a chopstick, you can also use a crochet needle or other long thin object so long as it is not so sharp that it might puncture your fleece. Position them at about 10 o'clock and 2 o'clock, raw edges aligned with the edges of your hat crown piece, and continue with the pattern as usual. Enjoy!