I'm answering my own question because I did a bunch of stuff from a lot of resources, and collected valuable information that more people may use in the future when coming across this post.
I wanted to use household items.
My flask is a stainless steel with some copper.
What I actually did:
Cleaned with a small amount of tap water, shaking it a little bit;
Boiled a glass of water, and a bottle of vinegar/water (50%/50%);
Cleaned the flask with the boiled water, them with the vinegar solution (50% of the flask, shaking it a little bit).
Then cleaned again with the boiled water (50% of the flask), shaking it, them with the vinegar solution again, then with the boiled water (50%), then with boiled water again (this time 100%).
I didn't emptied the flask from this last cleaning;
I let the water inside for 24h; 24h
later, I opened the flask, and emptied it in a glass cup: there wasn't any solid particles, and the water didn't have any taste, so I believed it was ok to put some whiskey.
Maybe your flask is an old one, with leather, I don't know.
In this case, I suggest you check these resources: http://www.whiskywhiskywhisky.com/forum/viewtopic.php?f=16&t=4254 http://www.thetfp.com/tfp/tilted-knowledge-how/29045-cleaning-inside-antique-hip-flask.html http://ask.metafilter.com/111820/Mildewsmelling-hip-flask-can-it-be-salvaged http://forums.overclockers.co.uk/showthread.php?t=18080329
cooking.stackexchange.com/users/14191/Somebody still uses you MS-DOS
If there's no evil smell emanating from the flask, just give it a good rinse with hot water.
Otherwise, a 20 minutes soak in a dilute solution of bleach (around 1 tbs per gallon), followed by extensive rinsing with warm water should do the trick.
cooking.stackexchange.com/users/None/
Is the flask actually stainless, or merely something that's been plated bright and shiny?I'd go w vigorously shaking some sand around inside of it, to loosen any corrosion/crud deposits.
Follow that w hot water/detergent, and brushing as possible.
Finally, give it a soak in Vodka/Everclear.
Check to see that the rinse solvent comes out clear and without chunks.
For all you know, someone from the 1990's left Cream liqueur in there to evaporate.
cooking.stackexchange.com/users/5455/Wayfaring Stranger
Steradent tablets are best for cleaning any type of flask in my experience.
cooking.stackexchange.com/users/5489/Echilon
There are a number of flask and bottle brushes available online that will help with this task.
I also found this set of instructions at ehow that recommends the use of boiling water and distilled white vinegar.
Unlike most cleaning products, white vinegar will not leave a lingering scent in the flask.
cooking.stackexchange.com/users/12565/Kristina Lopez
I have always had a lot of success using uncooked rice as an abrasive and a dish soap/hot water combo.
Shake it like crazy and it's clean.
It's a safe, simple, and effective method.
cooking.stackexchange.com/users/14859/Jason
Great question and some really good answers.
I am the founder/craftsman of a company that sells flasks and
we recently wrote a blog post on it that can be found here: http://1776.co/blogs/journal/54690755-how-to-clean-your-leather-whiskey-flask
The short of it is: Use warm soapy water and allow it to air-dry.
Your flask should always be washed by hand because the chemicals in dish washing detergent may react with the stainless steel interior.
cooking.stackexchange.com/users/37872/Isaiah Jones
I found an old stainess steel flask that someone once gave me. I want to use it again to hold whiskey.  There doesn't seem to be any liquid inside, though it may have contained some just before the flask was forgotten.   How do I properly clean to start using it again? Just hot water? No soap? Can I trust it? It was only used to hold whiskey.
How to properly clean a whiskey flask?
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<whiskey>
cooking.stackexchange.com/questions/28236
cooking.stackexchange.com/users/14191/Somebody still uses you MS-DOS
Rinsing with boiled water and then a white vinegar solution will not leave a lingering scent, or you could try hot water and detergent (possibly with uncooked rice as an abrasive), vodka/Everclear, Steradent tablets or just hot water if there is no unpleasant scent. You can use vinegar or soap and hot water. You can also try soaking in vodka or dissolving a steradent tablet.
Use vinegar and hot water. Use warm soapy water. Try soaking in vodka. Try dissolving a steradent tablet. You can use rice as an abrasive.