Firstly, scientists must know which gene or genes are mutated, altered or missing. They must then form an acceptable replacement and insert it into the patient's body. There are a number of ways to do this.
DNA (33474)
Ex Vivo
This is a process where problem tissue is removed and subjected to gene modification in a lab and then replaced. This is a very difficult process and often requires a lot of DNA. This technique is often used to treat immune system related diseases.

In Viro
In this process scientist take viruses, sterilize and revamp them, so to speak, inserting a replacement gene and then infecting the patient with the virus. These viruses are called vectors. This is a common approach for gene introduction because genes directly inserted into cells are normally rejected or Incorporated into the DNA strand incorrectly. It is also convenient mode of transport to cells otherwise difficult to reach. There are four types of vectors

Retroviruses - these viruses carry RNA not DNA but can create a double stranded DNA copy of the RNA Genomes. These copies are then injected into the cell and integrated into the chromosome. Chromosomes are tightly wrapped structures of DNA that exist within the nucleus of the cell. HIV is a retrovirus.

Adenovirus - This virus has double-stranded DNA genomes. When incorporated into a cell the DNA does not join the host cells genetic material, it remains free. This means, however, that it does not replicate itself when the cell divides and therefore must continually be reintroduced into the patient to continue the therapy. This virus is normally the cause of respiratory intestinal and eye infections, and also the common cold.

Adeno associated viruses - These viruses containing a small single-stranded DNA are classified as nonpathogenic human parvoviruses; dependent on the helper virus. They insert their genetic material in a specific site on chromosome 19. They are very good and effective at this and have a very low failure rate. The body also rarely reacts with an immune response to this virus, unfortunately it has a very low carrying capacity and cannot deliver a great quantity of DNA substitutes.

Herpes simplex viruses - These viruses contain a relatively large linear DNA genome of double-stranded DNA which can carry 100 to 200 genes. They're particularly good at infecting neurons.

This is the creation of an artificial lipid or fat that has a sphere with an aqueous core. It passes its genes through the target cell's membrane which does not normally cause an immune response which might otherwise be a problem.

Stem cell modification
This is a process of modifying stem cells in the laboratory and implanting them into the patient to reproduce. Currently scientists have managed to enacted effective treatment using bone marrow cells. This is an attractive treatment because when done properly the gene therapy will be self-sustaining and there will be no requirement for constant injection and re-introducement.

Also see Ethics of Genetics

And Types of Gentic Diseases