The most important thing is to understand what the instructor expects. In some cases, a VoiceThread assignment merely means clicking on and viewing a VoiceThread your instructor created, like watching a video or viewing a PowerPoint presentation. In others, you make be asked to view and leave comments. In some classes, instructors create assignments where you are expected to leave a required number of comments and then click a 'Submit' button. You might even be asked to create a VoiceThread of your own and submit it. Read any instructions provided and ask questions if unsure what is expected.
VoiceThread has provided detailed instructions for these slightly different scenarios, including videos and screenshots: