Hi!! I know Kumirei already showed you the way to get the timeline installed, but you may also want to explore the userscript section:
There’s a bunch that are universally useful that can be found with the WaniKani App Store script, like the WaniKani Forums Review/Lesson Status (so you know how long til the next review while viewing the forms), or the SRS review level indicator (lets you know what level the review item is) or even the Lesson Cap script (to make sure you’re not overloading yourself - handy as your levels go up).
These aren’t the be all and end all, but they should cover a lot. You can personalize this over time - there’s a bunch of other userscripts available to make life easier (or you can make your own - if you know code and don’t see what you want anywhere 
) Don’t hesitate to ask if you have any questions!