Installing TickThreading

Install Forge, if you don't already have it.
Download TickThreading
Put it in your minecraft mods directory
Run minecraft, join a world once if using a client, else just start it if using a server
Close minecraft
Run PATCHME.cmd/sh in your minecraft directory
Have fun, if anything breaks make an issue report or contact me on EsperNet/Freenode IRC. nick: nallar

Extra Performance Hints For Servers

Your start.bat/sh should look something like this (replace 8192 with the RAM the server should be allowed to use, in megabytes):
java -server -XX:useSSE=3 -XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC -XX:+CMSIncrementalMode -XX:+CMSIncrementalPacing -XX:ParallelGCThreads=3 -XX:+UseCompressedOops -XX:+AggressiveOpts -Xmx8192M -jar server.jar nogui