How Do You Get a Certain DNA Sequence?

But one thing proteins can't do is make copies of themselves. ... Some mutations have no effect, and others produce new versions of proteins that ... if you know the sequence of nucleotide building blocks on one strand, you know ... DNA copying process, which goes on much faster in rapidly dividing cancer

I would first use PCR (polymearse chain reaction) to amplify/increase the number of DNA copies I have. When I had a sufficient amount of DNA I could do use a technique called Compliment VNTR. (I am going to give you steps to forensic applications which is used to match a certain DNA genome with the perps DNA.)
1.PCR- makes more DNA
2.Endonuclease- cuts up the DNA
3.Electrophoresis- Organizes DNA using charges (large to small)
4.Southern Blot- transfer smear of DNA to membrane filter
5.Hybridize membrane with hot probe (The hot prob is the radioactive sequence of the certain sequence you want which will be complementary to the ones in the large samples of DNA)
6.Audoradiography- Compares the complementary sequences.