To keep it simple, I usually start from SRC compiler directive of Keil. I write a skeleton of function and let Keil help me to create a skeleton in ASM itself. Afterwards I include asm file with full instead of previous .C file and let .H as it is.
Sometimes I only write whole C function and only make complete or partial hand-optimization with the help of asm/endasm directives.
Last but not least, everything about mixing C and ASM codes is described in Keil's manuals, so if you need to know more RTFM.
Jan