Genome by genome? Gee, that'll get difficult...
Well for starters, you'll want to recognize mtDNA is passed down mother to child, mother to child, all the way down. Men do not pass down mtDNA. Women do. mtDNA is not the same as the DNA on your X-Chromosome either. It gets kind of tricky.
After that, you'll need individuals who have had their mtDNA tested. AncestryDNA does not test mtDNA, and I'm hesitant to call 23andMe's mtDNA services anything near precise. Your best bet is to get the full mtDNA test from FamilyTreeDNA, chances are.
From there, you're going to get the results, which will give you your mtDNA Haplogroup. It might look like nonsense initially because you know your recent ancestors, not the very ancient ones.
mtDNA testing has three general regions that are tested:
- HVR-1: Having a match with 0 differences here means that you are likely related in the last 1,000-1,500 years to an individual. Yeah. Good luck finding your common ancestor.
- HVR-2: Having a match with 0 differences here AND on HVR-1 means that you and your match are likely related in the last 500-1,000 years. Still hard to find your shared maternal ancestor, but, not impossible.
- Coding Region: Having a match with 0 differences here AND on HVR-1 AND on HVR-2 means that you and your match are likely related in the last 250 or so years.
There are these things called mtDNA haplogroups which do not have the same designations as the ones for yDNA. Finding someone in the same broad haplogroup is generally easy, but narrowing it down to the more specific haplogroups and subclades with small to no matches in another story.
Certain sites allow you to compare two mtDNA files quickly and for free (Mitosearch) and others allow you to get a more precise mtDNA haplogroup.
All in all, it's a long, difficult process requiring you know your mother's mother's mother's mother's mother's... (five hundred years later) ... mother's mother's mother. Or as far back as you can go, really.
Impossible? No. More difficult than yDNA? Probably.
