Round steaks are tough cuts that benefit from tenderizing. Pounding them flat with a meat mallet is common, especially for cube, country-fried or chicken-fried steak. Some marinade ingredients also help tenderize the meat, but most only impart flavor to the outer meat. If you're using both techniques, pound first, then marinate.
Pound Then Soak
Pound the round steak with a meat mallet before marinating it. Pounding meat that's soaked in a liquid will make a mess. Also, marinating only imparts flavor to the surface and just below it. Marinade can penetrate all the way through a cutlet, or at least close to it. This makes for a more flavorful finished product.
How to Pound
To tenderize a round steak with a mallet, lay it on a cutting board and cover it with a large sheet of plastic wrap. With enough force to flatten the meat, pound it, starting at its center. Use a downward motion, but one that angles slightly away from you. Radiate outward from the middle.
How to Marinate
Marinate a pounded round steak for one to four hours. The more acidic the marinade, the shorter the soak time should be. Marinate in the refrigerator in a ceramic or glass dish. Use equal parts acidic liquid and cooking oil, along with salt and seasonings. For a tenderizing marinade, use milk, buttermilk, yogurt, papaya, pineapple or kiwi. Don't mix cooking oil with dairy products -- fruit juice or puree works, as does laying slices on the meat.