Make a 5 gallon batch of a good hearty, slightly on the sweet side beer. This can be a sweet stout, a strong ale, a barleywine, a DIPA or IIPA, a tripel, etc.. Two weeks before your bottling date, take an ounce of light american oak chips and place them in a small 4 oz. mason jar. Pour a few shots of your favorite single malt whiskey over the chips, screw on the lid and soak the wood chips for 1 week.
One week before bottling, take a small hops bag and fill it with the whiskey soaked wood chips. Add a food safe weight to the bag with the chips so that the bag will sink to the bottom. I have some large glass marbles that work perfectly for this. Just sanitize them in starsan or iodophor for a few minutes before putting them in the baggie of whiskey soaked oak chips. After everything is in the bag, open the lid on your fermenter and toss the bag into the beer. Let the oak chips soak in the beer for about 3-4 days, then start drawing off a few samples with a wine thief, turkey baster or an auto siphon. When the beer tastes like it has just a little bit more oak whiskey flavor than you'd like, it's perfect and time to bottle or keg.
By the time the beer carbonates up properly, it'll be 3-4 weeks down the road. The oak in the flavor will have had just enough time to tone down a little in the flavor profile, leaving your beer with a perfect tasting whiskey oak cask flavor.