Source of the guide is http://mirror.softcox.com/e398-to-ROKR/ 

1. Check your Bootloader version by holding down # and * when you power the phone on

2. You must ensure that under Boot Loader you have 07.D0.  If you have something else, DO NOT PROCEED

AT THIS POINT ENSURE YOUR BATTERY IS FULLY CHARGED, OR YOU MAY BE STUCK IN BOOTLOADER MODE WITH NO WAY TO RECHARGE THE BATTERY!

3. Make sure your phone is in Data/Fax mode under USB settings

4. Find the directory where Motorola PST is installed and run MultiFlashFlex.exe

5. Under preferences check the first two boxes

6. Under Enable Flash check the box and select the shx file of the Monster Pack that you will flash

7. Make sure under Enable Flex nothing is selected

8. Start your phone in bootloader mode, this is done by holding down the star * and hash # keys while powering on

9. Plug your phone in and wait for it to appear in the list (it may appear as S FLASH NEPTUNE LTE)

10. Press start!

Note: This guide is for already modded to 373 version phones! All 372 (stock firmware phones) need to flash 42_for_all MP first and then the MP that they want.