To give the leaves a richer color consider spraying them with Ranger's Distress Spray in Walnut Stain. Let them dry overnight.
Measure out the length of the twine to fit your mantel plus a little extra for hanging down the sides.
Add a drop of hot glue to the stem of each leaf and gently press them onto the twine.
Space out the leaves on the twine in roughly equal amounts.
Then go back and glue mini pinecones in between the leaves.
Hang your felt leaf garland on the mantel and enjoy.
Notes
You don't have to buy a package of felt leaves. You could find a simple leaf template and cut out your own from a few felt sheets.You don't have to spray the leaves with stain if you like the color. I only sprayed one side. Also, you may be able to find similar leaves in more muted colors online or in other craft stores. I bought mine at Michaels.You could use a ribbon or some other type of cording instead of the twine if you want.I have tons of mini pinecones in my yard but you could skip the pinecones and space the leaves closer together. Just plan it out first to make sure that you have enough leaves for the garland length.If you don't like working with hot glue try hand sewing the leaves onto the twine with a needle and thread.