Applying for a fiance visa (K-1) can be a somewhat complicated process, but if all the necessary information is provided it can greatly increase you chances of success on the first try. Below are the steps required for filing a K-1 a.k.a. “Fiance visa” application with the USCIS.

The first thing you have to do is download the necessary forms from the USCIS website. (US Citizenship and Immigration Services) This will include one form I-129F (Petition for Alien Fiancé) for the alien fiance and two G-325A’s (Biographic Information). One for the fiance and one for the petitioner.

The download links are here.
www.uscis.gov/files/form/I-129F.pdf
Instructions:  www.uscis.gov/files/form/I-129Finstr.pdf
www.uscis.gov/files/form/g-325a.pdf

The petitioner  must fill out the I-129F form and a G-325A. The alien fiance must fill out the other G-325A form. Print clearly using a black ink.
 
The petitioner must provide proof of U.S. citizenship. This can be a copy of your original birth certificate. Copy, front and back. All pages of a valid us passport or a copy of their U.S. naturalization certificate, front and back. Only one of the above is required.

The petitioner and the fiance will each need to write a letter declaring their intentions to marry within 90 days of their arrival in the United States and sign it.
Enclose two passport style photos (2"x2") of the petitioner and the alien fiance with each ones names printed of the back of the photo. Place each photo in a plastic bag and print the name on it also. Attach them to a full sheet of paper and include them with the forms.

You will need to provide proof that the two of you have met in person within the two previous years. This would include copies of plane tickets, passport pages showing entry into the fiance’s country and photos of the two of you together. Note: The meeting in person of your fiance must have taken place before the filing of the I-129f petition. Failure to do this will result in your application being rejected.

Provide evidence of an on going relationship. This can include copies of telephone bills, letters and emails.

If either party was previously married, you must show proof that the previous marriage or marriages have been dissolved. These can include, a divorce decree or certificate, copies of a death certificate if the marriage ended with the death of a spouse.
If either the petitioner or alien fiance has changed their name previously, you must provide a  copy of any marriage certificate, adoption decree, or court order that documented the name change.

Include a cover letter explaining why you are filing the I-129F partition and include a list of all included documents.

Include a check or money order for the filing fee. As of October 25, 2009, the fee is $455.00 U.S.
Your application must be filed at the appropriate USCIS office or it will be returned. A list of where to file is below. Check this against the current forms in case there have been any changes.

If you’re a resident of, Alaska, Arizona, California, Colorado, Guam, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Montana, Nebraska, Nevada, North Dakota, Ohio, Oregon, South Dakota, Utah, Washington, Wisconsin, or Wyoming.

You must file here:

USCIS
California Service Center
P.O. Box 10130
Laguna Niguel, CA 92607-0130

If you’re a resident of, Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Mississippi, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Oklahoma, Pennsylvania, Puerto Rico, Rhode Island, South Carolina, Tennessee, Texas, Vermont, Virginia, U.S. Virgin Islands, West Virginia and the District of Columbia.

You must file here:

USCIS
Vermont Service Center
Attn: I-129F
75 Lower Welden Street
St. Albans, VT 05479-0001

Notes: There may be additional documentation required depending on your particular circumstances. For example if your fiance has any children that they want to bring over. Follow the forms carefully and provide any additional information required. If something is in error or missing you will not be notified until you receive their decision and if it’s rejected you will have to make the corrections and start over with another filing fee included.