The two handed backhand

The backhand is arguably harder than the forehand. If you find this isn't the case for you then consider yourself lucky!
To start, the overall motion of the racquet should be similar to how I described the forehand. The backhand, unlike your forehand, is hit on your weaker hand side, (right-handed people will hit a backhand to their left, opposite for left-handed). Because the backhand is on the weaker side, many people find it much easier to hit with two hands to steady the racquet, and find it easier to make their backhand consistent and accurate.
You need two hands on the racquet for this shot, and this should be tried first in a normal grip. Start by holding the racquet as you would for a forehand, and then place your left hand right on top of your right, in a normal grip as well; your palm should rest on the opposite bevel, (the bottom bevel).
The two handed backhand, as demonstrated by Sharapova, is started by turning your shoulders so they point straight down the court, feet one in front of the other. Next, start the back swing on your racquet, placing it waist level or just below, behind you. Swing upward, pushing out with your left hand, finishing over your shoulder. Last, step through with your back foot and keep your racquet up until the shot is finished.
Hopefully, with a couple of practice shots, this will become as easy as your forehand!