Texmod is a program which lets you temporarily replace game textures.
There is a great deal of information on Texmod out there through a quick Google search so I will not write much on the subject.
As it relates to Mass Effect 3, it requires a special workaround to function with Origin.
I have included a batch file (which automates the manual process detailed below) and short instructions in the Texmod resource folder of my default femshep mod.
You can download it here.
1. Copy your Texmod.exe into the folder where your MassEffect3.exe is located (Program
2. Rename your Texmod.exe into MassEffect3.exe and the MassEffect3.exe into MassEffect3bak.exe
3. Through Origin, click the game icon to launch
4. Instead of Mass Effect 3, the Texmod program should load
5. Go back into your game folder and change the exe files back to their original names (Texmod.exe and
6. Go back to Texmod, load the desired tpf, and browse for 'target application' MassEffect3.exe
7. Click 'Run' in Texmod to launch the game
Sundance31us on the BSN has uploaded a nice detailed PDF instructional with pictures. Download is at the bottom of their files page. Check out their desk frame mod while you're there! http://social.bioware.com/project/8081/
For a much more in-depth description of how to create texmods check out Ottemis's Guide to Creating tpfs.