The best way to improve the flavour of your alcohol is to barrel-age it. It certainly isn't the easiest, and if you buy an actual barrel it can be pretty expensive.
I'm cheap. I'll admit it. I also like to think of things sideways. What I've done is to reverse the process. Instead of putting the booze in a barrel, why not put the barrel in the booze? It solves so many problems. You don't lose any of your delicious alcohol by evaporation (called the Angel's Share), you don't have to prepare and care for a leaky wooden barrel, you don't have to take up a lot of space and you don't have to spend a lot of money. How does that sound? It sounds to me like you can buy some more booze.
I've taken kiln-dried lengths of Ontario white oak, and charred them like the inside of a whisky barrel. That's it. Yes, I'm selling pieces of pre-burnt wood for you to soak in your booze. It sounds crazy. It sounds mental. It also works.
1) Put a bottle of your favourite liquor into the canning jar. You could use any canning jar you have at home.
2) Put one or more aging tablets into the jar and seal it.
3) Wait a couple of weeks. Yes, a couple of weeks. This isn't going to happen overnight, and anyone who tells you different is just selling you pre-flavoured wood.
4) Sample your aging product to see if you like it. If not, continue to wait.
All liquors are different, so the wait times will vary along with your desired level of aging. You can keep your new aging jar out of the light if you want to, but I haven't found it made much difference in the short amount of time required. I like to keep mine near my coffee maker so I can see them. Every morning while I'm waiting for the coffee, I just give the jars a gentle swish and set them back down again. Mmmmmm, coffee.
You should use some masking or painter's tape (not included) to keep notes on each experimental jar. Sample the flavour with a spoon. Record the date you started and any other relevant details like, I don't know, what liquor you're using.
I've intentionally made these tablets too big to fit into most bottles. Getting the tablet out of a bottle once you've reached the peak of aging would be really difficult.
Each aging tablet can be re-used multiple times. Let it dry out between uses and keep a record of what it was used for last. If using it again, this might be important. I keep mine in paper envelopes I can write on.
For fun, try boiling an unused tablet in some milk on the stove to make a smoky cream sauce for fish or seafood. I've even used them to age maple syrup. You may have seen that product on my site at one time. I also use them in a couple of our cocktail bitters.