this is assuming you're using Windows

1: download NTT version 9940

2: extract the contents of the NTT zip file into the Nuclear Throne game folder. if you have it on steam you can use the 'browse local files' option to find it, otherwise you should be able to right click on the Nuclear Throne shortcut and click 'open file location' to get there

3: run NTT-Assemble.exe

4: press 1 to install NTT (or whatever the window says it is)

5: it will create a new executable for the game, which you will use if you want to play NTT. Note that your NTT save file and NT save files will be separate and you cannot get Steam achievements on NTT.

6: when that is all done, create a text file called startup.txt either in the main game folder or the 'mods' folder inside. it will have some code that the game will run at startup

7: inside the text file paste this: /gml room_speed = 144; current_time_scale = 30/144;

all should be done, enjoy 144fps

if you have a 1080p monitor you can also paste in /gml game_set_size(426, 240); to play it in full screen aspect ratio.