> My problem is that I cannot see how control flow works in Forth, e.g. a simple if-then-else.

What made it click for me was http://www.exemark.com/FORTH/eForthOverviewv5.pdf, specifically sections 2.3 "Loops and Branches" and 5.3 "Structures". With a slight simplification, if/else/then branching is defined in 7 words.

Two primitive words, branch and ?branch (in python because I know it better than assembly):

  def branch():
    """ branch is followed by an address, which it unconditionally jumps to."""
    ip = code[ip] # Get address from next cell in code, jump to it.

  def branch_if_zero():
    """ ?branch is followed by an address. ?branch either jumps to that address,
    or skips over the address & continues executing, depending on the value on the
    stack."""
    if stack.pop() == 0: # Pop flag off stack
      ip = code[ip]      # Branch to address held in cell after ?branch
    else:
      ip += 1            # Don't branch, skip over address & keep executing

  : >MARK    ( -- A ) HERE 0 , ;
  : >RESOLVE ( A -- ) HERE SWAP ! ;

  : IF   ( -- A )   COMPILE ?branch >MARK ; IMMEDIATE
  : ELSE ( A -- A ) COMPILE branch >MARK SWAP >RESOLVE ; IMMEDIATE
  : THEN ( A -- )   >RESOLVE ; IMMEDIATE