There's the Mastering Sideways ROM & RAM course on MDFS:
http://mdfs.net/Docs/Comp/BBC/SROM/Mastering/

The first half covers service ROMs but the rest covers language ROMs.

Also, Bruce Smith's book "Advanced Sideways RAM user guide" is now free to
download:
http://www.brucesmith.info/advanced-sideways-ram-user-guide/

Might also be an idea to get the developer to sign up to the StarDot forum
as there's always someone ready to help with programming questions on there.

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Essentially, your ROM type needs to be %11xx0010 to indicate '6502 code', your entry point is at 8000 which should JMP to your main code where you must initialise the stack, LDX #&FF:TXS and claim BRKV at &0202/3. You then own zero page from &00 to &8F, main memory from &400 to &7FF and user memory from OSBYTE &83 to OSBYTE &84. You must talk to the system via the API calls at &FFCE to &FFF7.

You also need a method of selecting your language, so you need at minimum a *command service handler to recognise your selection command and tell the MOS to enter yourself. There is a sample minimal *command selection routine on the Wiki which uses the ROM's title as its selection command: http://beebwiki.mdfs.net/Simple_ROM_selection_command_code