It sounds like your vet has given you the go ahead to just learn however you can. They coulda offered to show you though.
They are located at about 5 o'clock and 7 o'clock just below the anal opening. They are hard to miss.
Externally:
Put your thumb on one and your index on the other. Hold a paper towel back there and squeeze. If you feel something in there and nothing comes out, they could be impacted and the vet would need to take a look.
Internally:
Insert a gloved, lubricated (KY Jelly) finger into the anus. Grab on between your thumb (on the outside) and index finger (on the inside). Squeeze and repeat for the other one. If your finger is in the way of the gland opening, you could block the liquid from coming out. So you may have to reposition while in there.
The demos online do a good job IMO. It isn't easy to cause damage. For some dogs, if you never start it, then they will never need it. They are glands, so they want to be filled. They can fill very quickly.