I think we started at 6 months with the doctor's blessing.
We started with cereal and he only advised us to go through all the level 1 foods before going onto the level 2s. Also, to only add one new food every 3 to 5 days to watch for allergies or other things. We noticed that Brian became very gassy with some foods, to the point that he seemed to become very uncomfortable. So, we did rice cereal for 4 or 5 days then added another food and then it was rice cereal and whatever that over food was for 4 or 5 days, and so on.
Doing it that way made all the level 1s last a while and by the time we were done with it, Brian was ready for the level 2s. After the level 2s came the 3s. At 9 months, the doctor said that we could give him little pieces of cheese and small bits of diced fruits.
Looking back on it, the only thing that I might have thought to do differently would be to save all the sweet stuff (bananas, apples, etc) for after the vegetables. Brian may turn his nose up at plenty of foods but he hasn't met a "dessert" that he hasn't liked.
As time has gone on, he has certainly gotten more finicky about the foods he will eat. There are still a few standards that we can always fall back on, lick bananas.
As folks like to say, your methods may vary.