I’ve been using Renshuu every day for over a year now. It takes a bit of work to set up and I don’t know of a guide anywhere but I’ll try to point you in the right direction to get started.
The way I use it (which I think is the recommended way) is to create “schedules” which contain a pool of items to study and which automatically track which items you need to study each day. Each schedule contains items from one of the following areas: vocab, kanji, or grammar.
There may be a quicker way to do this for new users who don’t already have a schedule but this is how it shows up for me. You can set up a new schedule by selecting Learn>Study Libary in the main menu. On the left side of the Study Library, select “SCHEDS”, click the “Make a Schedule” button, and choose the type of schedule you want (vocab, kanji, grammar). Now give it a name and finally you have a schedule that you can start adding items to. You can add items from existing lists like JLPT levels, Kanji Kentei, textbooks, etc. Or you can add them individually from the site’s dictionary by clicking on the + icon and checking off the schedule you want to add the term to.
Once you finally have some items in your schedule, click the study button on the schedule to get started studying. You’ll probably also want to dive into the schedule settings at some point to change the quiz size, quiz types, number of new terms to add per quiz/day, etc. in order to get it set up the way you want to use it. You can change your timezone and daily reset time from your account settings along with a number of other options.
Hopefully this gets you on the right track. There’s a lot of customizability and it unfortunately isn’t very clear.