Download the release you want (cute.zip / cute.7z not the source)

Extract the cute folder into /.minecraft/versions/ or extract the content of the cute folder into /.minecraft/version/cute/

Add the new version in the minecraft launcher

Then just run the release like normal, if the game crashes and you have updated the version try deleting the /.minecraft/cute/ folder, it's likely a settings related crash

Open the Eclipse folder as the workspace directory in Eclipse

Hit Window > Preferences > Java > Installed JREs > Add > Standard VM > 'Path to java 8' and then choose the checkbox next to java 8 to use it

Delete the Server option on under package manager if it shows up (you only need client)

Drop down Client, right click Referenced Libraries > Build Path > Configure Build Path select 1.8.9.jar and hit Edit, then choose jars/versions/1.8.9/1.8.9.jar, then apply and close

Same as above but scroll down until you see Realms, hit edit, and select jars/libraries/com/mojang/realms/realms-1.7.59.jar

Try running using the green arrow at the top, I think everything should work

If the step above doesn't run the game, click the little dropdown arrow next to that green button, double click Java Application, then set project to Client, and main class to Start, then under the arguments tab put -Xmx1024M -Xms1024M under VM arguments. You should then set working directory to other and paste ${workspace_loc:Client/jars}. You're then good to hit save and run.

Assuming you've done the setup for Eclipse and can run minecraft from it, hit File > Export > Java > Jar File > *Drop Down Client* > Check 'src' > *Uncheck Everything Else* > Finish this will get you a jar file

Once you've done the above step, open the jar file with 7zip/winrar and delete start.class and META-INF

Then you need to find and open the 1.8.9.jar file with 7zip/winrar, should be located somewhere in the /.minecraft/versions/1.8.9 folder, and copy the assets folder, log4j2.xml and pack.png into the jar file you made

Lastly, in the repo you need jars\libraries\io\netty\netty-all\4.0.23.Final\netty-all-4.0.23.Final.jar, open this with 7zip/winrar and copy io into the jar you made with Eclipse, once you've done that you're done, and it should work following the install guide above