From Halopedia, the Halo wiki
There is no set process to gain skill, but that your skill is akin and relative to other players currently playing Halo 3 in Matchmaking. You do not gain skill unless you become appreciably and significantly better. If you develop a greater headshot ability, your skill level should rise noticeably, unlike with EXP, which only changes if you win a match, it takes longer to continually increase your skill level. Though skill is earned much quicker by winning free for all games, than by playing team games, you still must be able to get better at your game. It may become harder the more skill you earn, yet there is always something to improve on. You can practice on only killing with headshots, or trying to get five assassinations per game. No matter what you try to improve on, your skill should increase the more you excel.
Note: If you plan on bringing lower skill players such as skill 20-30 and 40-50 make sure they have a lower kill death ratio and win ratio (both can be view on Bungie's web site). Also be sure that they are able to fight the better players. It is suggested that they maintain a -3 to +3 Kill Death Spread to avoid them from leveling up too quickly requiring you to find another low ranked player.
However, there are complications to getting up the ranks:
Skill and EXP
Skill Level and Experience are measured in the Matchmaking system. Players will be rewarded with experience Rating Points (RP) earned for winning a game in Halo 3 Matchmaking. Progress from recruit to officer is a reward for many wins and another way for games to be evenly matched.
Experience Rating Points and their associated rank status (Sergeant, for example) will give other players an instant feel for how long you’ve been playing and how that compares to your relative skill level in a given playlist. A player with many RPs and a low Matchmaking skill level might be new to that specific matchmaking playlist. Knowing that is the key to knowing an enemy or teammate.