First of all, you have to know everything about your guest speaker. It is extremely important for you to get as much detail as you can about his work, family, friends, hobbies, interests and affiliations. It will help you in judging that whether he is the right person to get invited to your function or not. Furthermore, it will give you the necessary information which you can use in your invitation as an admiration note for his work.
You have to take a high-quality paper to write your letter. It will be better to use your letter head if you are running any kind of organisation. However, if you don’t have a letter head, you can take a simple piece of paper.
Start writing the letter with traditional salutation and use the title and name of the guest speaker for instance ‘Dear Dr. Jack’ or ‘Dear Miss Rose’.
Now, write the subject of your letter in which you have to give the briefly request him to join you as a guest speaker for the particular occasion. For instance, you can write ‘Humble request to join us for the...’.
Then, in the first paragraph, you will write the full request to the speaker with the detail about your function. You have to include all the necessary details about the function which you are going to arrange e.g. what will be the motto of the gathering or who else is invited?
In the second paragraph, you have to tell your guest that why you want him at your function in detail. You can use the information you gathered in the Step 1 to relate it with your objectives of the gathering. For example, if you are holding a business conference, you can call a successful entrepreneur to deliver an inspirational speech.
You have to end you letter by repeating your request again and adding sincere remarks. In addition, you have to mention your name and designation along with your signature.