How do I antique new painted wood?

What you're looking for is called dry-brushing. It is used all the time in the movies and theatre to make stuff look old. What you need are 2 small cheap paintbrushes. The kind that have funky bristles. They cost about 99 cents at The Home Depot. Then you'll want to find an acrylic craft paint the colors that you think matches best with the antiquing on your mantel. You will want both the cream and the darker umber.

Assemble your tools: paint brushes, a roll of paper towels, cups for thinning the paint, something to stir the paint, and water.

I can't tell the length of the window valance, so you be the judge on how much paint you're going to use. Stir the 'cream' paint in one cup with the water until it is pretty thin. It should drip quickly off the stick. Paint the entire valance with the thinned color and let it dry.

Now, thin the darker brown color the same way. You're going to need some paper towels in your hand as well as some clean towels on a surface. Dip your second brush in the dark color and wipe it several times on the flat paper towel until it seems 'dry'. Now brush it in the nooks and crannies of your valance. Immediately wipe off any excess with the paper towel in your hand. Continue until you get the look you like. If you overdo with the darker color, just go back with the cream to cover it. Voila!

You could try finishing it with dark wax. Besides giving the antique look you are after, it gives a great silky finish:

https://canarystreetcrafts.com/tips-for-using-dark-wax/

The easiest way is to use a dark glaze focusing on the inside edges of the moulding. The product is easy to work with and will make your project look as if it came that way. You can purchase this online (I got mine from Etsy) or at a big box store (Home Depot, Lowes, etc...)

You can wipe a wood gel stain over it, then wipe it off with a soft dry cloth (old men’s White t shirt works well). Leave more stain in the grooves and recesses. You control how much you leave or take away.

yes i agree with essie, use a dark wax and wax on, wax off! its found with the chalk paint and its brown. it will give everything a browner hue and when you wipe it off, everything thats in the recessed parts stays a bit darker

https://www.youtube.com/results?search_query=how+to+antique+wood

You can buy any finish you want to do this at a Michaels or AC Moore and recently I also found at a Joannes Fabric store as well. It is an antique finish in the acrylic paint sections that you rub on. However what I have found to be even easier lately was a product by Rustoleum that is a Glaze. You simply brush it one as you would paint-very lightly and then you remove it you want with a dry brush or a rag. It slightly darkens any color and stays behind in others leaving it looking distressed. Good for any color and a quart costs about 13.00 and seems to be lasting forever. I did my kitchen cabinets and a table and still have half the quart left.

Try antiqur buffing gel from the craft store

I LOVE, LOVE ANNIE SLOAN Chalk Paints & techniques. Pricey, takes time, but soooo worth it!

I would say use a glaze and wipe down lightly after applying so only a bit stays behind. You might even want to light sand some of the edges to give i a little character. You can also try using a dark wax using th same technique of wiping down lightly so some of it goes in the nook and crannies. Try practicing on a piece of wood so you you get the hang of it.