Making a quick note of what you are grateful for does wonders for improving your sense of wellbeing, and in turn helps you feel good, reduces stress & anxiety, sleep better, and ultimately improves your mental and physical health.
Keeping it simple makes you more likely to actually do it. My advice is to keep it handy: maybe in your bag, a desk drawer, on your sideboard, beside the kettle or on your bedside table. Go easy on yourself. You only need to write one thing, maybe even one word, per entry; and nothing bad will happen if you don't write in it every day.
Personally, I don't record the date of each entry and quite often just write one or two words. I keep my gratitude journal in my healing room, where I go for quiet time.