Get a journal. Any notebook will do, whether it's a college ruled note pad, a fancy journal or a guided gratitude journal.
Set aside time. Decide when you'll practice your new habit and carve out the time to do it. I like writing before I go to sleep - it's a perfect time to reflect on the day and drift off with positive, grateful thoughts.
Make it a habit! Set an alarm on your cell phone or leave yourself a note so you'll remember to write in your journal. It will become a habit eventually, but until then you may need a reminder!
Keep up with it. Some days will go badly and you might not feel like you have anything to write, but remember - THERE IS ALWAYS SOMETHING TO BE GRATEFUL FOR!! Isn't that wonderful??? Even if it's the clothes on your back or the food you ate that day, there is always something.