When can I help another player in Dark Souls 2 with boss fights?

When can I help a player with a boss-fight?

Does "My Boss" have to be alive, and were is good practice to put ones summoning sign? The nearest bonfire or just infront of the boss?

So long as you have the white soap stone you can begin helping other players whenever you want. You just use your white soapstone at a location and it'll appear in the worlds of other players at the same location, provided they are within 25% of your soul memory and the difference between your level and theirs is no greater than ((your level + 10) * 1.10). The location of your summon sign in an area isn't of huge importance, it just depends on whether you want to help them clear out the area first or not.

In order to be summoned by another player you need to leave you mark on the ground using a white soapstone. Look in your inventory if you already have one and then use in the area near the door leading to a boss (it doesn't matter if you have already killed the boss or not). Doing this will make your sign visible to all other players in the area who now have the option to summon you.

Always leave your summon sign as close to the bonfire as possible (whithin reason). There are a few exeptions though. Take capra demon, hosts/players can come from different directions/areas before they arrive at the white light/boss gate, so it would be difficult for you to get good chances to join a host actually wanting to kill the capra demon. In front of the entrance to the centipede demon is a good place to leave your summon sign, because of the bonfire after killing demon firesage, is so close, so hosts can only come from that direction. Just make sure you have the orange charred ring first, so that you can help your host the best way.