You need images to make an animatic. Animatics are made using sketches or 'scamps' - rough versions which indicate what's happening in a frame. Your images just need to convey action within each scene. Generally, these are the same images used at the storyboarding stage.
Scamped images in a storyboard
Remember, the goal of your animatic is to create a rough version of your final piece, not the final piece itself. Avoid using 'finished' images. Focus on timing, not visual detail. There'll be plenty of time for that later.
If you have a voiceover or soundtrack, that will define the length of your animatic. Equally, sometimes the duration will be determined by the final medium. For example, a video for social media often has to be made to a given length. Either way, setting the overall length of your animatic gives you parameters to work with when timing out your frames.
Historically, animatics were made by filming still images, then editing that film to the required length for each scene. Things have moved on, and now they are created using a dedicated animatic software.
If you've already made your storyboard in Boords, you can quickly switch from storyboard to animatic mode with the click of a button:
Turning a storyboard into an animatic with Boords
If you're not using Boords, you'll need to choose some software. Many people use tools like Photoshop or After Effects to create animatics. Both of these are more than up to the task. That said, the complexity of both tools means there can be quite a learning curve if you choose to go down this route.
Whatever animatic software you use, your animatic will be created using some form of timeline. Timelines are a visual representation of the duration of your animatic. The position of each frame on the timeline is marked, in this case with a yellow dot:
An animatic timeline with a frame selected
Moving these frame markers changes the duration of each frame, a process known as timing out your animatic.
Movement within animatics is shown using arrows. For example, this frame uses an arrow to indicate the small character jumping onto the large character's back.
Using arrows to indicate character movement
Using arrows stops you falling into the trap of animating too early. The goal of an animatic is to create a general sense of timing, so stay away from granular animation references. Arrows are your friend.
Arrows are also used to indicate camera moves, like this dolly out transition (as used in the famous Jaws scene). There is a world of directional techniques out there but use them sparingly.
The Dolly Out transition
Some images take time to process. You may choose to use a longer frame duration if you're showing the audience a particularly complex scene. Holding a little longer on a frame will give them time to take in details they may otherwise miss.
An animatic's strength is in its flexibility; they are not finished pieces. Therefore, it's easier to cut sections of your film based on feedback. If you have committed time and energy into creating a finished film, cuts like this are harder to make.
If you're using an online tool to create your animatic, there may well be built-in commenting (as is the case with Boords). This makes the process of gather frame-specific feedback much more manageable.
An animatic is a stepping stone. When you're happy with the timing of your film, you'll want to move into production. Your animatic then becomes your reference for your final film.
At this stage, you'll want to get your animatic out of whatever program you were using, and into the software you're going to create your final piece in. Boords allows you to export animatics as an MP4 file and also has an After Effects plugin which will enable you to import timed out animatics directly into your project.
With an animatic in hand, you're in the best possible place to begin creating a masterpiece!