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: