Advice Needed On How To Send A Letter Of Invitation To A Friend For A Visit

hi everyone,
Please, i need a guide on how to go about this.A friend of mine wants me to come for a visit to his counrty. what information do i need to give him concerning letter of invitation, how is the letter of invitation drafted,and every other thing needed like where should the letter be directed to and in what way etc.,.i will really appreciate ur response to my need thanks.

Normally in germany ,The friend will have to go to the forgin office and collect a form. Your name date of birth address etc he has to sign that he will come up for any expenses you may cause during your stay.