For the computer based HSK exams, the test materials and the time are all the same as the paper-based exams, you just need to click and type on the computer
For example, for HSK 4, you just need to click and choose the right answers for the "Ture or False" and "Multiple Choices" parts and you need to type characters for the writing part. There is nothing special to worry about.
The only thing you need to prepare before the test is checking the headphone and keyboard, and remember to check the Chinese input software.
Hope this will be helpful. : )
Li Ying