While the demo content and example programs still in development, you can start learning to use this mod by examples.
... or join discord community on the page https://mods.factorio.com/mod/fcpu/discussion
Tutorial videos
Very good tutorials was made by BLU12 Gaming, see his YouTube channel.
Example 1
It simply enumerates signals from red input wire and copies them in availabler registers from reg1 to reg7.
fCPU code:
clr
:loop
mod reg8 cnr
mod reg8 7
inc reg8
mov reg@8 red@8
sst reg8 reg@8
mov out1 reg8
jmp :loop