Tools
TXD Workshop or any IMG editor
Steve M's collision editor
Any text editor i.e notepad
Extracting files
After installing SAMP, open SAMP.IMG with any IMG editor, i.e TXD Workshop. Select everything and extract to a new folder on your computer. This should contain DFF, COL, TXD and map files IPL and IDE.
DFF
A DFF can be extracted right out of the IMG and readily used
TXD
Not all SAMP models use custom textures - a lot of them use textures from the base game, so check their model name in SAMP.ide or dev.prineside in order to locate the .txd file
COL
All SAMP models uses custom collisions. They are stored in .col archives which can be opened with Steve M's collision editor. The collision is named the same as the DFF model. To extract it, select it and right click 'export single'.