To make a designer baby, there are many steps and processes that have to be successful first....
These steps include:
1. Creating an Embryo with In-vitro  fertilization (IVF).
2. Removing a single cell from that embryo within
  the first 5 days of creation 
3. Genetically testing the cell.
4. The parents decide whether to discard the embryo
  or implant it in the mother's womb.

The genetic selection of Embryo's is called Pre- implantation Genetic Diagnosis (PGD). this is a popular way to select an embryo so parents can:
Check for diseases such as Huntingdon's disease and cystic
fibrosis or genetic conditions  such as Down's syndrome.

Create a healthy baby to treat a sick sibling.
 
To select the sex of a future child (this is
  illegal in the UK).