1. First stack the multispectral bands except the thermal (band 6) and panchromatic (Band 8).
In your case may be 1, 2, 3, 4
Using ERDAS: Image Interpreter ->Utilities->Layer Stack
Now you have one multistektral image with 30 m and panchromatic (in Landsat ETM Band 8 ) with 15 m.
2. Fusion the stacked image with panchromatic
ERDAS: Image Interpreter -> Spatial Enhancement -> Resolution Merge
Select high resolution input as panchromatic image
Multispectral as stacked image
There are several chose for methods you can tray which one more suitable
I hope it will help