Cooking a roast in a slow cooker is a very forgiving process. Times don't have to be exact, as long as the roast is done. In fact, cooking a little longer, even after the roast is done, is desirable for a fall-off-the-bone-tender roast. Cooking times will vary with the size of the roast and the slow cooker temperature settings.
Begin with a thawed (not frozen) roast. Browning the roast is not necessary, but browning develops the flavors, caramelizes the surface and helps the final coloring. To brown the roast before cooking, preheat a heavy pan over medium-high heat. Add enough oil to coat the bottom of the pan. Flour the roast, if desired, and place the roast into the hot oil. Brown the roast on all sides, then remove it to the slow cooker to finish cooking. Flouring the roast is not necessary, but it does help the browning process and will help to thicken the gravy a little in the end.
Cooking on Low
If you are not in a hurry, use the low setting. A 4 lb. roast will take a total of 6 to 8 hours total cooking time on low. The cooking time will depend on your slow cooker model and its age; the older the model, the closer to the 8-hour cooking time your roast will be.
Place any aromatic vegetables such as onions and celery on the bottom of the cooker, and then place the roast on top of them. Add liquid ingredients and flavorings. Set the temperature to low, cover the cooker and walk away. About 2 hours before dinner, add any additional vegetables such as potatoes or carrots that you wish to serve with the roast.
Cooking on High
If you are in a hurry, you can use the high setting. On high, a 3 to 4 lb. roast will take about 4 hours. Prepare the roast the same way, but set the temperature to high. Cover the roast and leave for 4 hours. Resist the urge to check on the roast during the cooking time; lifting the lid will lower the heat and increase the cooking time needed.
Tips for Tenderness
If you want your roast to be very tender, follow these tips. Use the low temperature, slow cooking will increase tenderness. Once the roast is done to an internal temperature of 170 degrees, continue cooking for another hour. When done to 170 the roast will still be firm and sliceable, but cooking for another hour will break down the tendons and make the roast truly tender.
Most cooks desire this tenderness when cooking in the slow cooker, as opposed to a firm slicing roast, but this is a matter of preference. If your roast is not tender to your liking, cook it a little longer at a low setting. Don't cook it so long that it is dried out; the times listed here should be sufficient, but they are very forgiving. Another hour will just develop the tenderness a little more.
Adding acid with the cooking liquid also increases tenderness. For this reason, many recipes include acidic ingredients like tomatoes or vinegar. However, if you cook the roast slowly and a little longer, acid is not necessary.