Save the game, preferably in the daycare to avoid advancing the RNG.
Note: Egg RNG in BW2 is different than in BW1. IVs are generated first, then the PID. If you're interested in only one part, you can skip to the relevant section.
Part 2: IV + Nature RNG
Important: If you are going for Nidoran or Volbeat/Illumise, this step will determine which species you get. Enter the correct egg species, and you'll know the gender of the egg (male for Nidoran M/Volbeat and female for Nidoran F/Illumise).
In contrast to Gen 6 Egg RNG, you'll RNG a seed and IVs for the next egg without needing a specific advance. Only hitting the initial seed matters.
Input the IVs, nature, and ability you want in PokeFinder (don't add other filters yet).
Search for an initial seed.
Start the game and hit the initial seed.
Talk to the old man and reject the egg.
Walk around until you have another egg ready.
Save next to the old man.
Note: If you only want IVs and not a PID, you can grab the egg. The RNG is done.
Part 3: PID RNG
Remove all the IV filters and input the PID filters (gender, shininess).
Search for an initial seed that gives your PID.
Start the game and hit the initial seed.
Advance the RNG until you reach your PID spread.
Talk to the old man and accept the egg.
Note: Sometimes with PokeFinder, you may need to talk to the man one advance earlier than your target. If your target is at 100 advances, talk to him at 99.
You should now have the egg with everything you wanted!