Its just like flying standby normally I except you have a bit more info.
If it is your airline, check the computer for boarding totals. If it isn't, ask the gate or call their non-rev reservations number if they have one.
To list on your airline you can usually do it yourself. For ID90 travel you have to get tickets from your travel dept and list with the airline ticket agents.
To get through security you use your company ID.
Then you wait at the gate and if there is room you get a seat.
Generally the priority is pretty low. Deadhead crew and revenue standby go before you. Employees of the carrier and possibly regional and code shares ride before non-company people. The rules for employees sometime have seniority or date of hire rules, some are time of listing.