Chances are you own at least one blank journal. People have probably been giving them to you as gifts for years. Now you finally have a use for one of them! :-)
There are at least four reasons why creating a Done Journal will help you in your writing career. For one, it shows the IRS or other taxing authority that you are not a hobbyist. You are actively pursuing a writing career and are officially in business.
But on a more personal level, recording everything you get done every day in your writing business will help you feel you actually achieved something - especially as you approach the end of the year each year. You can scan back over the entries and notice how much you got done! Good on you!
What you write can also help you plan for the future. What you've done and how long it took in the past will give you a basis for estimating how long things will take in the future. How long does it take you to write your average first draft? How long to get to a final, publishable draft? How long does it take your editor to get back to you, and for you to incorporate changes? How long does it take your cover artist to get you a final cover? Even things like how long it takes you to get ready to go away to a conference and how much time you need when you get back before you're up and running at 100% again will help you create more accurate planning calendars.
And not least of all - you need to celebrate your successes! You shouldn't wait a year, or seven or ten years, to celebrate finishing your first book. Or your next book. You need to at least have someone who will give you a high-five that you met your 2000-word writing goal today. And it does your neural pathways good to get positive reinforcement about the things you're excited to have accomplished. Your brain will try harder to do more of what you reward it for doing. :)
So go find a cool blank book or lined journal, write My Done Journal inside the front cover, then write today's date and the things you accomplished writing-wise today. At the very least, you can write down "Listened to writing podcast and started new Done Journal" on the first line!