I've been pretty hot for the idea of booze-infusions since hitting Rubi Bar in Barcelona last year, trying some of their 100 flavoured gins, and nagging the owner to tell me how it's done.
It appeals to me on so many levels; I actually get chance to use some of the kilner jars I've accumulated; I feel like a 1920s bootlegger without the risk of a Still exploding in my basement (or syphillis exploding in my boy-basement); I get to quote the Beer Baron episode of The Simpsons in my head - specifically "You forgot one thing chief...I filled the balls with a funnel" - and then I get to turn up.
The mechanics of infusing is pretty straight-forward - you put something, anything in with some spirits and the high alcoholic content preserves it for long enough for the flavour to come out and taint the liquor. Think of it as reverse-pickling - you know when you finish a jar of pickled onions and the vinegar you've got left is the good shit? Imagine if the vinegar was liquor, and it had been pickling fruit or spices or charcuterie instead of old onions.
If you're really going to get into it then Niki Segnet's Flavour Thesaurus is a really useful resource for infusing - as it is for many other things - in that it lets you pick out individual notes and flavours from a drink (Juniper in Gin, for example) and then find ingredients which complement or contrast with it, letting you make educated experiments rather than expensive misfires.
First-timers could do a lot worse than Jalapeño Tequila, a face-palmingly simple combination of Mexican flavours which is good for sipping on its own, or versatile enough to be used in a few cocktails. (In true meat-head bootlegger spirit, make sure you spell Jalapeño completely wrong on the label...)
Spoiler alert: The secret ingredient is jalapeños...