Once you have the tools you can have a look at the already made levels to get inspiration.
Edit one of the files named lvl??.inc and BombMMP.asm either with Assembly Studio 86
or any ascii editor (the notepad for instance).
lvl??.inc is the level, there are 3 parts: the name and some extra information,
the description of the bombermen at the beginning of the game and the level tilemap with
some comments to be able to read it.
In the BombMMP.asm file the important part is at the very end. Just after 'FakeFAT:' there
is a list of every level to use. After, for each level, two lines say the name and the name
of the file.
To do your own level copy one of the lvl??.inc file and rename it. Edit it and change
the data to fit your idea.
There are 8 lines beginning
with .db and followed by 12 hexadecimal numbers for the columns. Each number
represents a block of the level. Refer to the table just before to know
to what block a number corresponds. Don't forget that all the numbers are in hexadecimal, if you
calculate a decimal number convert it with the calculator of Windows. The last thing to do
is to add your level in BombMMP.asm .
Once done, compile your new file and send it to your calculator (and to me). That's all folks.