There are big and small ways to "Pay it Forward."
I try to make conversation with people who work in stores. A small conversation that humanizes the person taking my money may make their day easier and may put a smile on the face of the next person to come through their lane. Easy for me to do.
I make it a practice to let people go before me if they're trying to get out on a busy road. A small courtesy they may pass on.
I always say thank you when someone does something nice for me, perhaps holding a door open for me. That thank you can be encouraging so they continue to do nice things for others.
My large "Pay it forward" is raising and training service dog puppies. My Rocco is ready to leave me and go to his forever home as a companion to a young man who is pretty much homebound due to cerebral palsy. Rocco will learn to bond with him like he does with me and want to be right there, touching him if possible. I'll cry when he leaves because I love him, but this young man needs him, and I just want him. And I'll eventually raise another puppy for someone else. It will be a good life for Rocco because he'll have a person focused on him, someone worthy of his unconditional love.