Kindness is a generosity of spirit. It comes to life when we give of ourselves and our time to be of help to others, without expecting anything in return. When you show kindness to somebody you bring out the best in yourself, and a side-effect of brightening up somebody else's day is to feel happier in the moment yourself.
Pay attention to the impact your behavior has on others, and notice your own feelings in association to their reactions. And think about how you feel yourself when somebody else shows you kindness. What you give comes back to you in even greater quantity. When you are kind, you not only get an immediate payback in terms of a feel-good factor, you will also receive kindness from others, and in completely unexpected and unrelated ways.
It is so easy to find ways to be kind to others: say something supportive when you instinctively feel someone needs to hear it; offer help without being asked for it; smile encouragingly; swallow your criticisms; listen without judgement; let mistakes slide instead of assigning blame; make small sacrifices for the benefit of somebody in greater need.
A great rule of thumb I apply to my life is to do unto others as I'd have them do unto me, and, do unto myself as I'd do unto others. The latter part is just as important as the first. It is no good to be consistently kind to others and forget to be so to yourself. You will run out of steam, and feel less able to show kindness to others if you don't replenish your own mind, body and spirit on a regular basis.
Being kind to yourself means getting your needs met, being gentle with yourself instead of critical when you feel you're not performing at your best, forgiving yourself when the need arises instead of beating yourself up. When you get into the habit of treating yourself with kindness, it becomes much easier to extend that consideration and behavior to others.