https://wiki.speedsouls.com/SpeedSouls_-_Save_Organizer - a tool used to make savestates. create them before every boss or every crucial section for easy practice.
https://wiki.speedsouls.com/darksouls3:Practice_Tool - program that lets you give yourself infinite HP, give bosses infinite HP etc. for practice
https://www.cheatengine.org/ - if for some reason you want more functionality than Practice Tool offers, get this.
https://livesplit.org/ - the best timer
https://wiki.speedsouls.com/darksouls3:In-Game_Time_Livesplit_PlugIn - livesplit plugin that reads DS3 memory and automatically pauses the timer during loading screens

example runs:
https://www.youtube.com/watch?v=ua94OeCl4EU - Nemz38 WR run
https://www.youtube.com/watch?v=rCR-r_o265Q - squillakilla's 2nd place run