Injector o-rings are a fairly simple job. Things to keep in mind:
Be sure to keep the injectors in order and put them back in the same holes.
Be sure that the copper washer comes out with the injector or, if it doesn't, that you remove it before you reinstall the injectors.
Lightly lubricate the injectors and o-rings with motor oil before you reinstall them.
Check over your wiring harnesses for ANY damage as this is a great time to replace anything that is damaged.
Ohm your glow plugs for the same reason. If they are bad, they'll be at infinity or somewhere above 2-3 ohms.
Use quality parts. Don't skimp and end up with cheaply made Chinese-manufactured parts.
Don't give in to temptation and put those injector tips up to a wire wheel. Very bad idea. If they are seriously sooted up, use a rag with some carb cleaner.
When you remove the injectors, each will allow some amount of engine oil to drain into the associated cylinder. There are numerous ways to get this oil out of the cylinders prior to attempting to start the engine. **You MUST get as much of this oil out of all the cylinders as you can.** If you don't, you'll end up trying to compress a liquid (engine oil) which doesn't compress. Lot's of bad, ugly and expensive things happen that way.
When I put my Stage 1's in, I pulled all of my glow plugs then turned the engine over by hand using a socket on the crank bolt with a long handled ratchet. Oil will squirt out of the glow plug holes and can drip everywhere so have some kind of catch pan or card board, etc. under the engine.
Be sure to use an inch-pound torque wrench when reinstalling the hold down bolts. 125 inch pounds is pretty easy to do on an inch-pound wrench, 12.5 foot-pounds is a whole lot harder to hit each time.
If you gather all of the parts first and have an average ability under your hood, this can easily be done over a weekend if you take your time and are working alone. A Saturday or Sunday if you are good and/or have a helper.
Oh, I removed my hood first too. Way easier to get to everything that way.