depends on who you are and what angel(s) you are wanting to contact. for instance i contact a number of them on a regular basis for a number of things. i am part angel i guess you could say but its not necessarily just that... all i do is just focus and then say "I call upon 'Joe' to speak with me". joe is an example of course...
might work differently because i frequently contact them. you could look up some rituals for a particular one. i know there are some.
any more questions let me know.