This is something easy to do, and pretty cool at that due to the virtually limitless combinations you can concoct. For the sake of simplicity when I type this, I'll put up the recipe I made last: raspberry.
Here's what you need:
Raspberries (duh). About two big handfuls of them
Sugar or Splenda*. 2 cups
Water. 1 and a half cups
*Note about using Splenda, if you want to cut the calories way down, you can use Splenda, no problem. HOWEVER. Make sure you buy the Splenda that's specifically designed for baking. The regular stuff you get in packets to put in your coffee is high grade pure Splenda, so it cooks differently, not to mention that it's 600 times sweeter than sugar. That's no hyperbole.
Here's what you do:
1) Fill up a pot with your water, crank up the stove
2) Toss your berries and sugar/Splenda into the water, stir it around, let it boil.
3) Once it reaches the Boiling Point, turn down the heat and let the berries simmer in their own sweet jacuzzi for about 20 minutes.
4) When 20 is up, move the pot elsewhere and let cool down to about room temp. (I forgot to mention, this is a good time to grab a frosty beverage if required)
5) Once your syrup is cool, strain it into a bottle of your choice. You can use anything, I normally use a gatorade bottle with the label ripped off. Or you can use a wine bottle if you feel like it.
6) Put it in the fridge. When it gets fridge-temp, you're good to go.
You can use this mixture in lots of stuff, the most obvious one being drinks. Vodka, fizzy water and a touch of this sizzrup is delicious. Also, feel free to use literally any fruit you can think of, or combinaitons thereof to make different flavors.
I used this recipe with cracked black pepper actually. One table spoon of whole black peppercorns, and one table spoon crushed black pepper. It's an interesting flavor combo to say the least. If I were to do it again, I would say to use less sugar. I DID however, spread this syrup on a seared ahi tuna steak and it turned out superb. I highly suggest it.