Hint: IF, THEN, and ELSE are words that are partially evaluated at compile time.

IF remembers its own location and reserves space for a jump; THEN compiles a conditional jump if NOT true to its location, in the place where IF was, unless there was an intervening ELSE, in which case it will compile such a conditional jump to ELSE's location at IF, and an unconditional jump to then just before ELSE. So a full use might look like:

    : TEST > IF ." greater" ELSE ." less or equal" THEN ;

    void test(x,y) {
        if(x > y) {
            printf("greater");
        } else {
            printf("less or equal to");
        }
    }