Here, you can see a model of LST estimation using ERDAS :
http://giswin.geo.tsukuba.ac.jp/sis/tutorial/koko/SurfaceTemp/SurfaceTemperature.pdf
This is a simple model, assuming that Emissivity is uniform (=1) and no atmospheric correction is done (no corrections for atmospheric transmission, upwelling and downwelling radiances). So, if you apply this example, you can obtain a relative LST, witch is not the real-ground LST, but is very used in most literature.
For more details about LST algorithm, search for http://www.sciencedirect.com/science/article/pii/S0034425712004749