You can start using website by clicking "Create an account" at the top right. Then follow instructions how to go forward.
You can download exercises or the whole theme as PDF. To do this, click on the radar icon at the top right of a domain. After uploading the file, you can print it or send it forward. 
No, this website consists of seven themes, and it is entirely up to you to decide in which themes or exercises you want to do. 
No, you don't have to. This website consists of seven themes, and each theme consists of three parts. By clicking on one of the themes you will end up in this topic and you can navigate to the different parts. You can always choose which theme and which part you click on and you do not always have to complete the entire theme. So you choose which exercises you go through and in what order.
Yes, you can always adjust your answers. Please notice, that if you want continue using the website later and adjust your answers in different themes, you can only access your account by using your personal link. If you lose your personal link, you don't have an access to your account anymore. Then you have to create a new account.