If a student is using an iPad, they can download the app from the App Store (or it can be pushed to the iPad if it's a school iPad). If the student is on a Chromebook, go to app.seesaw.me in the Chrome browser. The initial login page will look like the picture below.
They will click "I'm a Student." It will pop up with a window that prompts the student for either a QR code, or a text code. Use whatever way is implemented, or the way that was taught to you. This code will allow them to join a class. If you don't see a student who was added to your class, refresh your page. The students, once added, will pop up in the right-hand side of the screen. By default, a student can see everyone else's work along with their own. This is a setting in the Student's section of Class settings that you can toggle off if you don't want them to see anything other than their own posts.
In the event that the student is logged in a simply wants to join a new class, they can go up to their profile picture and name, and select 'Join Class.' The teacher needs to provide the class code to all students who need it. What the student will see is a large window with a field to enter the class code, and after they hit submit, they are part of that class.
In the Activities tab, they can only see the activities that they have not submitted a response for. In other words, the Activities tab serves as the student's "To-Do List." When the student is in the Journal view, they do have the ability to make a post (a photo, drawing, file, etc) that is not attached to an activity. The teacher has to approve this post before it can be viewed like any other posts or comments.