Prepare the cranberries- there are two methods, one was to prick holes in the berries (sterilize a needle and pierce right through in a couple places), the other was to freeze them (clearly easier). As in notes above, pricking brought out more cranberry flavor so would recommend if you have time.
Half fill one or more sterile bottles or sealable jars with the cranberries.
Add any additional flavoring/ aromatics- we did one with orange rind, another orange and a cinnamon stick, another with lemon rind and rosemary.
Fill the bottle or jar with gin and seal.
Store in cool dry place for at least two weeks, but a month or two will let the flavors develop a little better. Beyond that, you should strain the gin to store any longer to save the fruit going bad in the gin.
Once the gin has matured, strain the solids out of the gin.
Place the water and sugar for the syrup in a small saucepan and warm over a medium-low heat until the sugar dissolves. Set aside to cool. Combine syrup with the gin to get desired level of sweetness (I suggest 1 part sugar syrup to 6 parts gin) - you can either do this as you use it or all at once and store in it's sweetened state.