From: edbw.unixg.ubc.ca (Edgar Wickberg)
All homemade herb or vegetable flavoured oils have doubtful safety if they are not refrigerated immediately and kept in the refrigerator. The reason for concern is botulism. Clostridium botulinum, the organism that makes the deadly poison that results in botulism, grows in airfree environments. It is only when it grows that it produces the toxin. Putting anything up in oil produces a really great airfree (oxygen-free) environment and therefore the risk of toxin production. Clostridium botulinum is a very common organism in all of our environments, but the organism itself isn't dangerous to us. It requires, besides the airfree environment, a low acid one and temperatures above fridge temps. This is probably more than you ever wanted to hear about why not to make garlic flavoured oil. Why not dry most of your garlic so it will last and make just a little oil and keep it in the fridge. To make the kind that you store in the fridge, just heat a small quantity of oil (of your choice), put a few peeled cloves of garlic in a small bottle, pour the hot oil over, cool for half an hour, cover and refrigerate.
From: baker.325.magnus.acs.ohio-state.edu (gwen baker)
Take the cleaned herbs and place into a jar (a mason jar will do). Heat the oil to just warm (too hot and you will cook the herbs instead of extracting the taste), pour the warm oil over the herbs and let set. Check every 3 days to once a week, straining and adding additional herbs until the desired flavor is reached.
Henriette's comment: keep in refrigerator until desired flavor is reached, and thereafter.