Hi there! R18 as a scraping source is dead now, since they took down their store front. The itch.io version will need to be updated to support additional store fronts. Downloading from R18 still works, so users who have not backed up their libraries still have a chance.
Put all the Jav-It files and the additional files from this guide in the folder you want to use for your downloads.
Log into R18.com and export the cookies file. Rename it cookies.txt and put it in the same folder as the other files.
Choose the streaming option of the video you want to download and copy the ID from the URL.
Double-Click download bat.
Enter R18.
Enter the ID you copied before.
Wait for the download to finish.