Basically, it depends on how much fat is required and what it is being used for.
Essentially enough fat is taken from elsewhere by liposuction. This could be anywhere, but for convenience is often the abdomen or thighs and can be done under local anaesthesia or sedation. The fat may be processed by filtration and centrifuge spinning and put into small syringes for injection.
The Cadogan Clinic offers several exciting refinements, which your surgeon will discuss if indicated.
Fat contains Stem Cells and their close relatives, Mesenchymal Stromal Cells (MSC) which enhance fat transfer and improve skin and scar quality. These can be extracted in the Cadogan Clinic from the fat and added to supercharge the fat to be transferred. Alternatively, they can be used on its own as a stem cells suspension or MSC enriched Nanofat.
Finally, the fat can be stored for later or staged use, where larger quantities are needed, particularly in natural breast or buttock augmentation.
It does not stop there as so often patients are looking for contouring or reduction of fat elsewhere and this can be the ideal donor site - liposuction with a purpose.