Steam can be taken by either the convenient Steamer or from the boiling water on a heater. The second option is dangerous especially with kids.
It's not necessary to keep the face too close to container. Even a chair close to the steamer is enough. You can leave the steamer on and do your table work.
Should I put something into boiling water like vicks or karvol?
Steam is a pure form of water. Water evaporates after boiling and when it condenses into air passage, then it leaves moisture there. This moisture helps moving dried up small hairs which ultimately moves the mucus. Putting these things may increase the pleasant experience like smell but as such they do not have an additional function and may not be necessary to use.
Nebulizer is no substitute of steam. It has different function which can not be done by steam either.
Nebulizer is for lungs whereas steam is both for lungs and nose. Nebulizer is way of delivering drungs for local action in lungs while steam delivers moisture both to lungs and nose.
Some viruses like Rhinovirus which causes common cold are susceptible to heat. So taking steam helps in eliminating such viruses.