Mixins client base for 1.8.9
Clone the repository into whatever folder you want (Mine is in my coding folder), open the project in Intellij IDEA then go to the gradle tab on the right and run setupDecompWorkspace in the build/forgegradle, once that's done run genIntellijRuns, when that's completed a run configuration should pop-up named "Minecraft Client", copy the settings from the image below
Now you can start Minecraft!
Run the "build" command in the gradle tab, there should be a new file in Mixin-Base/build/lib, keep that folder open for now, now press Windows + R and type in %appdata%, this will open a folder called "Roaming", you'll see the .minecraft folder, open it, now navigate over to the "versions" folder and make a new folder inside of this folder called whatever you want, for me this will be "exampleclient-1.8.9", now copy and paste the 1.8.9.jar file from the 1.8.9 folder in the versions folder into your newly created folder (exampleclient-1.8.9), then do the same thing but with the exampleclient-1.8.9.json file I've left in this repository, you can rename this file to the name of your folder (also rename the .jar you pasted), now open the .json file with whatever text editor you want to use and edit the id to be the name of your file, now edit the "name" under "libraries" to say "net:example:exampleclient-1.8.9", then change the --tweakClass property to the path of your Tweaker class in the project, now in the .minecraft/libraries folder make a new folder called example: "me", "dev" and then inside that folder change it to the second folder specified in the json file, so my folder path will be net/example/exampleclient and then paste your jar file generated from the build cmd into that folder, rename the jar file to the name of the second folder, add a dash and then add the name of the third folder, so for me the file name is example-exampleclient.jar.
Open the launcher, start mc and it should be done!