This post has been updated on 11 October 2019.
I have done autosomal DNA testing through all four of the major testing companies (FamilyTreeDNA, AncestryDNA, MyHeritage and 23 and Me). I spend most of my time with AncestryDNA matches trying to get them to transfer their results to GEDmatch. This is to explain why and give me a place to give instructions without having to repeat them.
Why
AncestryDNA tells you only that you match people. It identifies the common ancestor by locating the same people in trees of the match who are also in yours. This may not reliable because you may be related in more than one way or you may have unidentified ancestors you share. Part of the reason we do DNA testing is to learn about ancestors we do not know about yet. The matching is only as good as the genealogical work (whether good or bad) that has gone into constructing the trees.
The other companies (FamilyTreeDNA, MyHeritage, and 23andMe) give tools that allow matches to discover on what chromosome and on what segment on the chromosome people match. It is then up to you to search the genealogies and attempt to discover the most recent common ancestral couple. If two people match on the same chromosome and segment and they discover a common ancestral couple, they can hypothesize
that the segment came from a common ancestral couple.
I have tested several other relatives in various lines only through FamilyTree DNA. But I've uploaded results to GedMatch for all of those relatives. Gedmatch accepts raw data from all three testing companies. Therefore you get a much larger pool of potential matches. Increasing the pool of possible matches is a great benefit for people testing with all three companies, but AncestryDNA testers derive the greatest benefit because they can finally see the "segment data," i.e., on what chromosome and segment their matches lie. For some of us who have proactively tested multiple relatives, we also may be able to tell you that you also match one of those relatives and therefore narrow the match to a specific branch.
How
This involves two steps: 1.) downloading your raw DNA data from the testing company and 2.) uploading it to GEDmatch.
1.) Download raw DNA Instructions for AncestryDNA:
Under the DNA menu tab, choose "Your DNA Results Summary" from the drop-down menu.
Click on "Settings" under your name (or user name) to the right.
Scroll down to the "Actions" section at the bottom of the page. Click "Download."
You will be prompted to enter your password. Check the box, and click "Confirm." You will receive an email with a download link. If prompted, select the save file option, not "open file." Pay attention to where the file is downloaded to your computer. This is usually the "Downloads" folder unless you have changed settings. The .zip format of the file is needed to upload to GEDmatch.
For FamilyTreeDNA, from the home page, in the Family Finder section, click on the small orange text "Download Raw Data." Choose the Build 37 Concatenated Raw Data option. For 23andMe, from the menu to the right of your user photo and name, select "Browse Raw Data." Then click on the link for "download."
2.) Upload raw DNA data to GEDmatch.
Go to GEDmatch. After the words "Not Registered?" click on the link to create an account. Once you have an account, log in.
You will see this section for "Upload your DNA files" in the right half of the screen. Click "Generic Upload."
The next screen is self explanatory.
Insert name of the person who was tested.
Use an alias in the second box if you don't wish the real name to be used.
Choose the sex.
Don't worry about the mitochondrial haplogroup or Y haplogroup questions unless you happen to know them.
Select the appropriate testing company, and check "yes" that you are legally authorized to upload the kit and select the option that follows which gives you legal authorization.
Click on "Yes" to allow your data to be used for comparisons.
Click on the "Choose File" link to navigate to the location of the downloaded file from AncestryDNA or other testing company.
Once you've selected the file, click "Upload.
Follow the instructions for not navigating away from the following screen until the upload is completed. This doesn't take long.
You will be able to do one-to-one comparisons right away. So do tell the person who encouraged you to upload that you have done so, and give her the kit number. You will be able to see the kit number right away on the home page (the landing page after logging in.) She will be able to start comparing with those known relatives. It can take a day or two for the test results to be processed for doing the one-to-many matching. One-to-Many is the tool that shows your match list. It is similar to what the testing companies show, but with more information.