Consider the size and shape of the barrels that are used to age the spirits you are trying to emulate. Calculate the ratio of the inner surface area to the volume of the barrel. Based on some whisky barrels I've seen, I add about 70 square centimetres of oak per litre of 55% spirit. Keep in mind that one "strip" of oak has two surfaces that interact with the spirit. I age spirits in 4 litre glass bottles and add thin strips (<1mm) of oak that I had wrapped in aluminum foil and lightly charred on the stovetop. So if my oak strips are 2 cm wide I cut a total length of 70 cm, but I usually break such a strip into a few pieces before charring and adding them to the jug. Gives nice colours and flavours and generally a smoother drink.
Oaking - Several different flavors can come from a single type of oak if alcohol strength is adjusted during maturation. 55%-53% will give vanillins, 40%-50% will give a mix of vanillins and sugars, 40%-49% will give sugars.
What I like to do is start at 55%-53% for first phase (1 to 12 months) then dilute to 40% (3- 12 months). In this manner I am adding sugar from the cells of the wood while I marry the dilution water to the whiskey. This results in rich vanilla oak charater with silky legs that cling to the side of the glass. The procedure works well with all types (chips or BBL) and varieties of Oak.
If you can't buy the commercial toasted/natural oak chips for flavouring 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 furnature 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 aluminium foil, and bake them in your oven for a while. The temperature of the toasting will affect the flavour that develops...
Jack Daniels is 140 proof grain whiskey when it goes into the barrel. The barrel is made from American white oak from Missouri. Each barrel is roasted at 450 degrees for four hours then flash charred over a 1500 degree gas flame for 4 minutes. It is then steam quenched to "activate" the carbon (and put out the fire!). The roasting carmelizes the sugar in the wood before the flash burn creates the charcoal. The steam treatment not only puts out the fire, but it causes the charcoal to expand like popcorn creating trillions of tiny bubbles. This GREATLY expands the surface area of the carbon. A fresh barrel will have approximately 1/3 inch of char on the inner surface.
Jack Daniels Whiskey is aged in the barrel for four years. During this time the barrels are racked in wooden barns. There is no heating or cooling in the barrel houses. The heating and cooling of the natural weather cycle in central Tennessee causes the whiskey to expand and contract with temperature. As the whiskey expands it is forced into the wood.In the wood it dissolves carmelized sugars, and tannins. As it contacts, it moves out of the wood and back into the barrel. The carmelized sugars give the whiskey it's red-brown color. The whiskey will penetrate approximately 3/4 of the thickness of the barrel stave. The penetration line is easy to see on an old barrel stave. Jack Daniels uses each barrel only once. After the whiskey is removed, the barrel is sold to other whiskey makers, European wine makers, the makers of tobasco sauce, and the public.
After ageing the whiskey is diluted to 80 proof before bottling. To answer your question regarding toasting and charring... They are not the same process. Toasting is baking the wood at 450 to carmelize the sugars. Charring is actually burning the surface of the wood to create charcoal. For hobbiests, it is easier to use toasted chips for flavor, and activated charcoal filtering for clarifying. These two processes are combined into a single stage process in barrel ageing.
Charcoal that can be used to smooth the flavor of bourbons and rums may be made at home like this: use either hickory, oak, or sugar maple wood (buying it from a lumber supplier is easy enough) as these are proven to be nontoxic.
Split the wood into finger- width sticks about 4 to 6 inches long, then pack them into an old coffee can that has no more of that coffee smell or any rust. Pack them in standing upright so there isn't much space between them. Once the can has a solid layer of these sticks crammed in together like sardines standing upright, cover the top of the can with a layer of heavy tinfoil that has a pencil sized hole in the middle of it. Place this arrangement on your propane burner (this is not to be done inside!!), and set the heat on high. After a bit of heating up, some steam, then other various flammable organic gases will evolve off (if the foil swells up, make the hole a bit larger- try not to burn yourself). Once there is no more gas/steam coming out, turn off the heat and let the can sit outside to cool on it's own with a cover to exclude any air from getting in. Once cool, rinse any ash off in some cold water and use however you wish. Do not poke any holes in the coffee can - that will allow air into the mix and turn all of the wood into ashes, instead of turning it into charcoal.
Using a pressure cooker to inject steam could also be attempted to try an make activated carbon, but I haven't tried it.
I see people trying to duplicate Jack Daniel's Tennessee Whiskey (It is NOT a bourbon - the charcoal filtering procedure was legally recognized as producing a separate type of whiskey after World War II) on some of the e-groups; the essential step is to filter the unaged spirit through charcoal (not activated carbon) that has been made out of sugar maple wood.
The residual sugars in the charcoal give a slight sweetness to the finished spirit, and the limited absorbing ability of the charcoal only makes the stuff smoother, but doesn't strip all the flavor out. I have tasted homemade whiskey and rum made and filtered in this method, even at "barrel strength" (75%) they were very smooth - no ice was even needed.