so how is it achieved?

There are various methods to Sequence dna, some more accurate and/or  faster then other but achieve the job NONETHELESS.

Gel electrophoresis

Gel electrophoresis has been around for a very long time, it is the oldest and most widely used method for dna SEQUENCING. this process involves loading dna into a gel pad and running electricity through it. the dna will move ACROSS the gel, heaviest/biggest sinking and lightest/smallest rising to top.

COMMERCIALLY available dna sequencing kits.

Over the years there has been a great rise in dna SEQUENCING demand and interest. various companies are now BEGINNING to offer dna SEQUENCING COMMERCIALLY. 

such as: 23andme, ancestry.com, familytreedn

23andme

most of these companies focus on simply providing ancestry/origin information rather than complex rich data. 23andme is the most information rich out of these dna SEQUENCING companies but still very lacking.

acestry.com

ancestry as well is nothing special, just gives you a brief overview of your origin and family tree. The family tree requires prior family members have taken the test or have some type of family tree document which renders it useless for a lot of people.

familytreedna

this is probably the most useful of the 3, it includes lots of information regarding your oriing and family. it also goes into detail of what makes you related to that certain person. 

further information

these are all great and all but they honestly don't provide any type of serious or in depth info such as predispositions, defects, and other stuff. well there is a fix for that.

promethease