Please give us more information about what experiment you are performing? What instrument are you using? What metals are you trying to determine?
Here is some general information:
Flame photometry is an example of "atomic emission spectroscopy." In this technique, metal atoms are aspirated into a flame. The solvent evaporates and the metal ions absorb energy from the flame, which puts them into a higher energy level. The atoms then release this energy in the form of light (atomic emission). Each metal has unique energy levels, and they release light at different wavelengths of light; each metal produces a unique spectrum.
If we have an instrument that can separate light into separate wavelengths (a monochromator), we can then see the light from all the metals in the sample. The amount of light that we see is proportional to the number of metal ions in solution, and therefore is proportional to concentration. This is how we determine the amount of metal present.
Sample preparation is very important. All organic material must be removed. This is done by digestions with strong acids.
If you have more questions, please write back.