Even though title says night light I would be kind of afraid of use them as that because they get really hot and could short out and what not. Also be very careful when working with mains voltages its not deadly but will deliever a very bad shock, and never place vaccum tubes close to anything flamable.
Step 1: Get Some Old Tubes!!!!!!!!!
Step 2: Finding the heater pins
Step 3: Powering the tubes?!?!
Most vaccum tubes run on fairly low voltages, so I hooked mine up to a variac,MAKE SURE AND DONT PLUG DIRECTLY INTO WALL AT FIRST. Slowly adjust the voltages to you start to see tubes light up. Mine did around 40-50 volts, but the tubes i used were quite small so if u decide to use power tubes or something like that it would probably take much more voltage. I did accidentally blew out one tube at about 80 volts, so when it gets a good glow going dont turn it up to much more.