Be sure your bike's front wheel axle is vertically over the front roller axle.

Definitely in a doorway, the narrower the better. Position the rollers so the center of the roller frame is in the doorway. Clip in with your normal foot. Hold the bar tops next to the stem with your hand on the same side as your clipped foot. With the other hand, grab the trim over the door opening with your fingertips as you mount. Clip in the other foot. Lean against one side of the doorway with your hands on the bar tops. If your rollers are in the right spot in the doorway, you should be able to push yourself upright by pushing against the door frame with your elbow. Start pedalling slowly while leaning slightly against your elbow. Try the equivalent of a 42X17 gear. Gradually increase pedaling speed, then push off with your elbow so you are balancing. Don't steer with the front wheel. Steer by shifting your weight. Just moving your head back and forth can be enough. Try to hold the front wheel steady. If you fall off, you'll come to rest against the doorway, though a pedal may gouge your calf. Most people fall off from time to time during their first week of rollers. Look ahead and try to relax.

Some people find it easier to start pedaling while balanced, with their hand still holding the trim over the doorway.