Think of one activity that you used to enjoy or that might give you a sense of accomplishment. Even a short walk counts.
Choose an exact time today. "I'll walk at 3:00 PM" works better than "sometime today."
Remove obstacles in advance. Have your shoes by the door. Reducing friction makes starting easier.
When the time comes, start. Motivation often follows action, not the other way around.
Rate your mood from 1-10. Compare it to expectations. Most find reality better than expected.