That depends on the video format. If they’re mp4, you only need to install any video downloader addon your browser (I use ‘video downloadhelper’ for Firefox and it works well for mp4). The trickier one is .ts files, which is getting more and more common nowadays. They basically split the video file into parts and making it a hassle to rip/download. If you’re good with computer, you may want to checkout JDownloader. It’s a command-based freeware that can basically do any download-related task. It can even rip a whole website if you know the command lines.
Otherwise use Internet Download Manager. It does its job well. At least for me. That’s what I used to download a bunch of Pornhub premium vids during their free month.