Beginners should start in a large area free of obstacles. Load yourself like a spring by crouching on the bike with extremely bent elbows, knees, and hips, rolling slowly along, pedals parallel to the ground.
From the crouch, explode straight upward, keeping pressure on both pedals and pulling upward on the handlebars with a forward twisting motion (reverse to that of a motorcycle throttle). The twisting motion helps bring the rear end up.
In the beginning, don't worry about which crank arm is forward and which is back. Just do whatever comes naturally. Then, after you get comfortable with the basic lifting, make sure to practice with either crank arm forward, and learn to direct the handlebar twisting motion so it turns the front wheel slightly left or right.
On the trail you'll want to be able to hop with either side forward, depending upon which side an obstacle is on, and you'll need to be able to turn the wheel toward the best trail line.
Once you can get over six-inch obstacles on a regular basis on flat terrain, you're ready to do the maneuver on a slope, hopping as you traverse across it. Start near the bottom of a gentle preferably grassy slope. Do a series of slightly upward and downward traverses, hopping forward, right and left as you go.
Note that on a slope, you want to keep your weight to the uphill side to keep the frame upright, and to keep from tumbling down the hill sideways.
Once this becomes comfortable, you're ready to use the gravitational pull of a slightly downhill traverse to begin mastering standing bunny hops. Rolling across the slope with the uphill crankarm forward, touch the brakes, bringing the bike to a quick momentary halt. Play with this maneuver until you are at ease holding the bike still for a couple of seconds without tottering.
Next, get in a mild crouch, and just when the bike stops moving forward, make a small sideways hop downhill, keeping the front wheel pointed forward or ever so slightly downhill. Release the brakes as you begin the twisting and pulling jump motion on the handlebars. After a landing/rolling exit becomes routine, begin pedaling forward as soon as your wheels touch the ground and straighten the bars to maintain your line on the slope.
Once you get that move down, expand the length of your standstill until you are no longer using the momentum of your traverse to hop. Then try hopping the bike upslope.
If you ever feel yourself falling, try to fall uphill. With luck, you've picked a spot clear of obstacles, maybe even one with grass on either side of the trail for a soft landing. Large fields or lawns are great places to practice bunny hopping.