Make sure you have imgtools or SAMI. I use imgtools.
Download the file, extract the taxi.txd and taxi.dff and before you fuck everything up, open imgtools, find the gta3.img file in GTA SA folder, extract the ORIGINAL files from it, save it somewhere, then replace the ones in gta3.img with the ones you downloaded. For extract and replace you right click on the file. To make things easier, click edit and then find then write in the white box the name of it.
I suggested to extract the original ones in case it doesn't work well and you want to put the original back on, a.k.a backup file.