What Janet and Geoff said, start on one knee.
Being big, heavy and stiff I found ukemi awkward to start with, Donovan Waite's video was very helpful
I also visualise my arm is a springy sapling
Get comfortable with forward rolls left and right before you worry about backwards rolls which I found much trickier.
Once you do get onto them the key is to remember your hands are going up to the opposite shoulder from the leg you first go down on and that is the shoulder you'll roll over. Also the feeling is not so much backwards as sideways/diagonal. Those were the two key points that helped me get it.
After a while you will find ukemi becomes one of the greatest pleasures in aikido, like a giant massage