1. Lace
You can wear a lace shirt, dress, skirt, or even a lace headband. I would recommend pairing lace with something less sweet and more "edgy" for a more interesting, rounded look.
2. Pastels
Pastels and neutrals are common colours for dresses on Downton Abbey. Or you could go with black. They always seen to be mourning someone's loss at Downton... But anyway. Pastels can be easily incorporated. Depending on the look you're going for, you may want to mix the pastels with a darker or somewhat brighter colour, or with another complementary pastel. Both add variety.
3. Hats
As you can see in the picture, straw hats go nicely with the Downton style. It might be necessary, however, to customize your hat a bit to make it work. You can dress the hats up or down to fit your personal style.
4. Shoes
Shoes usually have a heel, but are not too tall (I would estimate about two to three inches). You could wear these shoes with jeans, for a casual chic look, or with a dress or skirt, for a fancier look.
5. Hair
The hair styles may be a bit difficult to emulate, but barrettes and headbands like those worn on Downton Abbey are readily accessible, and easy enough to diy.
Note: All images were found through Google.