It will depend whether you are playing online or in a land based casino, as to how you set about entering a slot tournament, in fact in both playing environments you may find invitation slot tournaments or ones that are open to anyone that wishes to enter them.
The first thing you will have to do is to locate a casino offering slot tournaments, as not all of them do. Microgaming for example have their software all geared up so the operators of those casinos can offer the standard set of tournaments or configure their own.
If you come across a freeroll slot tournament then you are not going to have to pay anything to take part in them, for as their name suggests there is no entry fee and both online an mobile casino usually offer them each week, and in some cases each day.
You will need to register to take part in these slot playing tournaments and by doing so make yourself available at the starting time, you will then be allocated a set number of tournament credits and a set amount of time and must then play the tournament slot once the tournament starts.
There are no skills what so ever to winning slot tournaments, for the aim is to simply play off your credits as rapidly as you can do and hope that you win as much as you possibly can do with those credits and before the tournament timer reaches zero.
The winning credits you are awarded when playing off a slot tournament entry will then be turned into points, and the casino operator will have put in place a leader board on which all entrants and their scores will be listed and they will then allocate a prize to the players that score the highest number of tournament points.