If you have been assigned a task to mark, you will see this in your tasks section:

Click on the name of the task to open the marking screen.

You can navigate through the questions needing to be marked by using the unmarked navigation arrows:

Look at the answer the student has submitted and compare it against the marks scheme.  If the answer is correct, check the corresponding tick box in the mark scheme:

Questions with more than 1 mark available will have multiple mark scheme choices, follow the same process as above:

When you have finished marking all the questions, you will be greeted with a popup:

Click Release Marks and Release to finish.