Easy isn't always best.
Also, it is important to make sure you aren't dealing with something serious, which you might make worse by following some amateur advice. Having said that, here's my amateur advice.
Simply to relax the back, the best position I know is lying on your back on a firm surface, knees pulled up a bit, hands on chest, elbows on (or near) floor. To get the most relaxation from it, you need to hold the knees - and the arms - with some external force, so they don't have to "hold" themselves. A band of cloth or rope that goes around the two elbows, adjusted to the right length, will hold them in place for you. Similarly, the knees can be held in position with a band of rope tied at the right length, or just place a cushion or rolled-up towel under the knees to raise them.
For a long time I thought I needed to round my back (stretch forward) to ease my back pains, and that's what I did a lot - stretched forward. A physical therapist told me to do the opposite, and it has helped. Lie on the stomach and arch your back while pushing up on your arms. Do it a number of times, resting a few seconds in between.
Remember, get expert evaluation first!
"This advice is not meant to diagnose, cure prevent or treat any disease!"