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.
• DocJade programmed a single assembler with fCPU to beat Factorio.

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

Example 2 (complex system for train management by Cid0rz)

https://github.com/cid0rz/CTS-Cidorz-Train-System

Example 3 (recursion)

clr
mov r1 10        ; n = 10
call :fib        ; fib(10)
hlt

; int fib(int n)
; r1 = n
; return r1
;
:fib
enter 0
    ble r1 1 :fib_base

    push r1
    ; fib(n-1)
    dec r1
    call :fib
    mov r2 r1     ; r2 = fib(n-1)
    pop r1

    push r2
    ; fib(n-2)
    sub r1 2
    call :fib
    pop r2

    ; fib(n) = fib(n-1) + fib(n-2)
    add r1 r2
:fib_base
leave