This is an updated version of Fintech's guide. It worked fine until I tried to get the screen rotation to work correctly. Below is what worked for me. I have tried this on both RP3 & 4 and on 3 different XPT2046 screens purchased over the years.
Not much documentation, just the cut & paste that works for me
Create octopi image
Image Write Imager_1.6.2.exe
clt+shift+X to add wifi credentials
Step 1. Enable SPI by using the terminal command:
sudo raspi-config
sudo reboot
Step 2. upgrade Raspian
sudo apt-get update --allow-releaseinfo-change
sudo apt-get update
sudo apt-get upgrade
sudo reboot
Step 3. Now to modify the configuration file to configure the display
sudo nano /boot/config.txt
#then add this line to the bottom
dtoverlay=piscreen,speed=16000000,rotate=90
sudo reboot
Step 4. Log into you Octoprint and install the touch UI plugin.
#Run the following command to install X and touch screen auto login/boot into touch mode
sudo apt-get install xserver-xorg-input-evdev
git clone https://github.com/BillyBlaze/OctoPrint-TouchUI-autostart.git ~/TouchUI-autostart/
sudo ~/TouchUI-autostart/helpers/install
sudo reboot
Step 5.Install the screen drivers.
#This will fix the touch touch coordinates
git clone https://github.com/waveshare/LCD-show.git ./LCD35-show
cd LCD35-show
./LCD35-show
sudo reboot