It is estimated that around 80% of the flavor of bourbon and whisky comes from the oak barrels used to store them. We can replicate those flavors by soaking our spirits with oak chips or shavings. Start by using one teaspoon of oak per litre of alcohol, and let it soak for a week. Taste test frequently to find the level of flavor intensity that suits you - e.g. maybe a little more oak, or longer, or different % alcohol, or different levels of oak toasting.
If you can't buy the commercial toasted/natural oak chips for flavoring and aging, you can try making your own. Make sure you use oak or non-resinous wood - using a soft resinous pine will only give you a retsina. Be ingenious when looking for old oak - locals here use bits of old furniture etc. (after shaving off the varnishes etc). Smoked manuka timber is particularly good.
To make your own toasted timber, find a tin with a push on lid of 1-2L. Split your timber into thin enough strips to fit your bottles. Light the pieces, and when well charred, place in the tin. Place the lid on lightly to snuff out the flames. Add more wood as it becomes ready, replacing the lid each time. When cooled, push the lid on tightly to retain the smokey aroma until ready to use.
Another way is to wrap the oak chips/shavings in aluminum foil, and bake them in your oven for a while.