Barrel conditioning is really a matter of personal taste, and I predict we get many different responses here. So what follows is IMHO!
Prep: make some stillage so the barrels can sit on their side. I don’t worry about sanitizing them, the just had 120+ proof whiskey in them. Try to ask for "freshly emptied" when getting them from the distillery. They may "seep" a bit but after 6 barrels used, iv'e not had a real leaker yet. Use good sanitization during the transfer and filling for hoses ect.
Time & Temp: If I had a cellar (55f) I would put them there but I have had good results @ room temp 70-75f. Make sure your beer is fully fermented first, thus strong in alcohol and not much yeast. I leave mine in the fv for at least 4 weeks. Flavor will change overnight but I would plan for at least a month. My Barleywine is a yearlong process w/ at least 6 months in the barrels. That’s the fun of this, the monthly taste testing! Also fill the barrels all the way up, to avoid oxidization. I make a bit more than can fit in 2 barrels and put the rest in 2-3 kegs which go into the cold room for storage and are blended back in later.
If your going to Bushmills for the kegs say hi to my mate Kenny McConkey in the lab-- he worked for me as an apprentice here in the states.
Brewmaster, Figer's Brewhouse
"Your results may vary"