This quest is about as straightforward as quests can get. The first step is to head over to a capital city that belongs to your faction, and find the Festival Talespinner (Horde) or the Festival Loremaster (Alliance). You are informed that there are emissaries of the elemental plane, spirits of fire, near each of the bonfires around the world. If you wish, you can take some Summer Incense to one of these emissaries to honor them.
There are bonfires in just about every zone in Azeroth, including the newbie zones near whichever capital you've found yourself in. However, before you run out and finish this quest there, you might want to pick up your other Midsummer quests first and plot a more efficient route. You can probably hit a bonfire while you're passing through another zone for a separate quest. In this case, your best choice may be to do it in Ashenvale while completing Unusual Activity. Be careful on PvP servers, as many of these bonfires will be a warzone since there are prizes to be gained for desecrating the other faction's flames.
Once you've spoken to the Scorchling and given him your incense, you'll be treated to a fun little scene and, of course, your reward: A single Burning Blossom.