If you have a 7.3l, then its easy as pie. I think you need a 10mm deep socket. Once you see the new glow plugs you'll be able to figure out how they are put in. The job is similar to changing spark plugs. Be sure to take out the glow plugs though, not the injectors. You'll be able to tell the difference because the injectors are connected to rigid fuel line, while the glow plugs are connected by wires which slip off so you can put the socket on them.
GLOW PLUG ARE NOT LIKE SPARK PLUGS!!!! THEY ONLY NEED TO BE REPLACED AS NEEDED! I DONT KNOW HOW TO TEST...BUT I DO KNOW IS THAT IF ONLY ONE OUT OF 8 IS BAD THE TRUCK WILL BE HARD STARTING. YOU MIGHT SAVE SOME $ THIS WAY.. GLOW PLUGS ARE $25.00 PER PIECE
thelawnguy has it right. Glow plugs are not that expensive, but they will definitely make for hard starting. Burned up a starter with that one. No way would I pay 350 or 600 for glow plugs/installation. Check your local parts house first.
Easiest way to check a glow plug is use a test light.Clip the lead on the + terminal,yes positive,and then touch the test light to each glow plug.If it is good the test light will glow.Make sure you are touching the small centre prong on the glow plug,and not the body of it,or it will cause the light to come on,when the glow plug may actually be bad.This is the same as using a continuity tester,just much easier.The light will come on when grounded,so when you touch a good glow plug,it has a ground,and the light comes on.
BTW -as someone mentioned previously,anti-sieze compound is a must,preferably the silver colored,or it will never come back out.