Basically:

1: Make an image 128 x 64 pixels. Two colours and saved as a .bmp file

2: Use LCDAssistant to create the Hex Byte text type file.

3: Copy and Paste the Hex Byte data from LCDAssistant text type file in to _Bootscreen.h file.

4 : Modify _Bootscreen.h file other settings to allow for the new Hex Byte data and image size.

5: Modify the Configuration.h file.

6: Upload and Flash the modified Marlin version 1.01.1 RC8 firmware as normal.