You can enable map voting by putting the mapvote 1 command to the sapp init file (which is found in the sapp folder, which is found in the gametypes folder).
Example: C:\Users\username\Documents\My Games\Halo CE\sapp\mapvotes.txt
Write mapvote options to the mapvotes.txt file (put it in gametypes sapp folder as well) like this: map:gametype:description
bloodgulch:Slayer:Bloodgulch - Slayer
hangemhigh:Clasic Slayer:Hang 'em High - Classic Slayer
dangercanyon:CTF:Danger Canyon - CTF
(Make sure that you have the map and the gametype, and wrote them correctly)
You can specify the amount of displayed mapvotes / round with the max_votes [number] command.
The default value is 5.