To print out your entire history, just go to your Export Settings and download the file with your whole history. You can open up and print the file using Google Spreadsheet, Excel, or just as a plain text editor.
You can also print out just part of your iDoneThis history, instead of the whole thing. Click and drag your mouse over a range of dates on your calendar. This will select a date range that you can export by clicking on one of the icons on the upper right hand side of the screen.
The icon on the left creates a plain text report. To create a PDF report, click on the middle icon. To create an email report click on the icon on the far right. You can then print your reports from the plain text, PDF or email file.