From the main menu, go to 'Campaigns > Halo: Reach > Playlists' and complete three of these.
The shortest playlists are 'Self Preservation', 'Got Your Back' and 'Noble Team Doubles', each being three levels long.
Remember (for speed of completion):
· Grunts, Jackals, Skirmishers and Brutes can all be taken out pretty easily with Magnums, DMRs, Needle Rifles and Sniper Rifles in one or two shots - use the appropriate zoom settings for each weapon and keep your ammo well-stocked.
· Don't dilly-dally killing enemies that you can just speed past on a Ghost or other vehicle.
· Hunters are easily distracted by your Hologram ability and the rest of your teammates (of whom the important ones are invulnerable) and are susceptible to Magnums and Assault Rifles in the lower back.
Completing the first playlist will mean you unlock the 'Reach Has Been Good to Me' achievement too.