Once you have installed Tasmota using the guide here;
https://templates.blakadder.com/sonoff_NSPanel.html
Copy nxpanel.be file to to device;
https://github.com/peepshow-21/ns-flash/blob/master/berry/nxpanel.be
Change the autoexec.be file to load nxpanel.be instead.
Copy your tft to a place accessable via http.
In tasmota console type;
flashnextion http://www.whatever.com/nspanel.tft
That should be all you need.
If you prefer to use NxPanel instead of stock, it's even easy
installnxpanel
That's it. If you turn on auto updates in the setup, you don't even need to reflash again. it will it all for you.