First start with a larger, focal piece- a mirror or art. You can place it centered or slightly off center and it can be mounted to the wall or resting on the mantle- just make sure its your tallest piece, closest to center. Next work your way out with smaller objects, they create the other two corners of the triangle.
In the photo below by Centsational Girl- she works out to the larger potted plant and apothecary jar then two smaller pieces at the furthest corners and then works her way back into the center with small pieces again- but never going above that center point made by the mirror.
Here's my parents mantle that I styled a few weeks ago using all Homegoods items. I started with a large mirror at the center and then went out to the sides with a candle stick and topiary and then created some layer by place the bird figurine in front of the mirror.
Keep the triangle in mind and you'll be styling your mantle like a pro!