I make quite of few of my own bitters, and liqueurs. And my own cocktail cherries, but my favorite time of year is when citrus is in season and I can restock my supply of homemade citrus infusions.
I already have two bottles going. Basically just fill a 750ml bottle with 50% Grain alchohol and 50% vodka. The grain is harder for me to get so I stretch it with the vodka. Straight vodka is fine if you don't have grain. Been using Sobieski on the vodka front, but anything reasonable will do. (Let's not restart the whole vodka debate for the 100,000th time, colorless flavorless liquid, as long as you are not buying total plonk they pretty much all taste the same, particularly for these purposes.)
The rest is easy. I have one bottle filled with the peels of some very nice satsuma tangerines, and another with the peel of some yummy tangelos. Having tasted both already they are quite simply amazing and far superior to ANY commercial product including Cointreau. Gran Marnier is a bit of a special case, but I prefer these even to Gran Marnier.
Once they have infused for 5-10 days strain the peels out. NEVER let the peels get exposed to the air. You'll kill the batch with nasty off peel tastes. I ruined a batch that way last year.
If I want to drink it plain I typically add a touch of simple syrup and serve it neat, or with a bit of ice.
In cocktails I use it straight in lieu of orange bitters or Cointreau, adjusting the sweetness accordingly.
You can use pretty much any peel or zest. On oranges with thicker skin I typically zest.
You can also use dried peel. I dry my own on the radiator, or a combo of dried and fresh for some very nice complex flavors.
Give it a whirl. Promise you will be blow away by just how tasty it is. My nearly tea totaling, 70-something year old Italian mother in law who has been making her own liqueurs since childhood (for company) couldn't resist a second thimbleful last night after dinner!
Oh, and once you make them, if they mellow in a cool dark place for a 1-6 months, some of the sharper flavors fade and more orange candy notes gain in predominance...
Sorry this post is such a mess, I keep adding, editing, really should redo the whole post.
Couple of other items. In the bottle ALWAYS keep the peel covered with booze! If the peel is exposed to the air it will ruin the batch. A few floaters on top is okay, but if you pile them so high that they are out of the liquid, add more liquid.
Also if you use grain, you can always dilute down with water by about 40% when it is done, and add sugar to taste.
Other notes: Don't do this with fresh bitter orange (great for marmelade), just harsh, or bergamot, which might make an excellent cologne (did I mention I make my own) but is not really that palatable as a libation.