To add a new Journal to your account, using your mobile app, you'll want to go to Journals from your Dash screen.
Once you're on your Journals screen, you'll want to click the "+" in the bottom right hand corner of your screen.
Clicking the "+" will open up a small options menu at the bottom of your screen. From here you can choose to create add a new entry or a new Journal. You'll want to select "Add Journal."
Once you've selected "Add Journal" a small window will open where you can name your new Journal. Once you have the Journal named as desired, press ok. The new Journal will appear in your Journals list.