Install TI Connect CE on your computer
Download a TI-84 Plus CE Tetris program file in .8xp format from a trusted source
Connect your TI-84 Plus CE to your computer with a USB cable
Open TI Connect CE
Drag and drop the .8xp file into TI Connect CE or use the Send to Calculator option
Select your calculator and transfer the file
Safely disconnect the calculator
On the calculator, press `PRGM`
Find the Tetris program in the list
Press `ENTER` twice to run it
If the game requires a shell like Cesium or MirageOS, install that first and then transfer the game file