U need to use NameHack... (there's a lot of tutorials in Youtube) is easy to use
But i don't like it, beacuse when u are playing Halo and you press F1, anyone can view your name.
So if u put ur name like this (example):
{XG}ŧŌńƳ117
When u press F1 and in the postgame carnage report ur name will say like this:
{XG}
Thats why i recommend using Alt Codes, cuz with alt codes ur name dont change
Alt + 228 = õ
Alt + 155 = ø
Alt + 157 = Ø
Alt + 190 = ¥
Your name with alt codes: {XG}tøn¥117 or whatever u want
More Alt codes: check this
http://www.elcodigoascii.com.ar/codigos-ascii/letra-l-minuscula-codigo-ascii-108.html