This is the most popular tool for importing and exporting textures and some models from the iff files used by 2K Sports. It only imports and exports dds files for textures, so be sure to read up on how to edit and use those files.
You need to make sure your Java is updated, be sure to only get Java updates from Java.com
Once it is finished extracting, you want to run the .jar file.
Once it loads it will ask you to choose what game files you are editing, I'll assume you will choose NBA 2K11 since 2K12 isn't an option.
Navigate to your NBA 2K12 folder or to where you saved the .iff file you want to edit. If you used the 2K12 Decompressor, goto it's main folder (see tutorial above for directions).
In the left panel choose which texture (or model) you wish to edit by clicking on the different numbers. Once you find the texture you wish to edit click [Export Res].
You will also want to note the information on the bottom left corner of the Mod Tool that describes the dds texture you have selected. In the image above the dds texture of the Boston Away Jersey Top has a DXT5 compression with a width of 1024 and a height of 512.
Unlike the 2k9 Iff Editor, the dds file you import back into the Mod Tool must be the EXACT same size and compression as the file you exported. This is covered in the DDS tutorial below.
When you are finished editing your dds file, or if you have a dds file already created, click the [Import Res] button to import the texture back into the iff file. The Mod Tool automatically changes the iff file so there is no undo or save command.
Quit the program and enjoy.
Also know that some files contain "Subfiles" or textures. You access these textures and 3d models click the drop down menu next to subfile.