First, you should decide which chunks should be slime chunks, and which should not. Clicking on a chunk on the map once will mark it as a slime chunk (green color), twice a slime-free chunk (light red). Another click will reset the chunk to its default state (= ignore). Above the map, you can see an estimate of how many seeds fulfill your requirements (1 in x). After running the app for a few seconds, you'll also get a rough estimate of the time it will take to find the seeds.
Searching for Seeds
Using the panel above the map, you can start, pause, resume and stop the search. Once you click start, you cannot reassign the chunks before stopping the search. You can also reset your configuration anytime, which will stop any ongoing search as well.
On the left, there are two sliders you can adjust. Seeds determines how many seeds you want to find before the search is stopped automatically. Workers represents the amount of web workers to use. The best setting for this depends on how much strain you want the app to put on your system, and on your hardware.
Once a matching seed is found, it will be listed in the right area. You can click each seed to directly view its slime chunks within the app. At this point, you won't be able to change your configuration until you click the edit button.
To scroll, use your arrow keys while your mouse cursor points at the map, or move your mouse while holding down down the left mouse button. You can use the slider below the map or your mouse wheel for zooming. The lower inputs allow you to go to a specific point (e.g., your base) of the map and set a marker there. You can always remove and add the marker by double clicking on the map. The "Save Map" button allows you to save the currently shown map as png image file.
Below the map you're also able to switch between PC and Xbox, which will change the way the highlighted chunks are determined, but only if there's actually a difference between the two platforms. Also, with Xbox enabled, the map will be limited to its actual size.