Click "Login to AniList" button to get redirected to their login page
After being redirected back to the website, use your mangadex credentials to log into mangadex
"Get follows" button will retrieve your follow list entries
(optional) "Get scores" to retrieve the ratings you gave on mangadex. If you don't click this, no ratings will be applied when porting to AniList
Click the final button to upload to AniList. Depending on how much entries you've had, it may take a while, since we need to take a breather between sending requests to not get rate limited by AniList
Currently there's no progress bar implemented but you could check the network tab in your browser's developer tools