This new-to-me blog had me with this image, which is a fav.
If you saw the pic above and thought "What?", then check out the more traditional Emily A. Clark. Ms. Clark loves restyling her mantel, and this link goes to an archive of her mantels over the years.
Probably the best instructional content I found was on Design Sponge.
While I secretly have a soft spot for the quote as graphic art, you should be very wary of utilizing it above your mantel. This, in particular, is totally not allowed. Like, I'll try not to talk shit about you but if pressed, I'll describe my overall hesitation about our friendship as "she had words over her fireplace, you know?".
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My advice is to start with how you want to fill the space, which generally breaks down to mirror, art, or dead animal head (with architectural fragment filling in a seldom well executed fourth option). The rest is about balance. Even an asymmetrical assemblage has to be balanced with the rest of your room in order to not look like an incomplete thought or storage. Here are some personal favs:
A symmetrical room with asymmetrical mantel objets by Jamie Meares.
Nanette Lepore's crazy Eva Peron portrait via Elle Decor (also note vases on one side balanced by the bust).
Rita Konig's taped photos from Domino (via)
Hollister Hovey's taxidermy and antiques
Hovey Design's lighter approach from their book, Heirloom Modern; horns/tusks are a great sculptural objet and less fussy than most vases and ginger jars. Just make sure they are legal.
And while I don't condone monthly mantelscapes (which, per my online research, is a thing), it is easy to experiment and change what doesn't work. Bon chance!