“If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:”  1 John 1:6

In the main verse he says we can’t walk in darkness if we have fellowship with him. Jesus had said-
“Then spake Jesus again unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life.”  John 8:12
So fellowshipping with him is following him; following and copying the word; talking like he talks and doing like he does, just as he thought us in his fellowship with the father.
“For I have not spoken of myself; but the Father which sent me, he gave me a commandment, what I should say, and what I should speak.”  John 12:49
“Then answered Jesus and said unto them, Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise.”  John 5:19
Also, fellowshipping with him means being where he is, in the same position as him:
“If any man serve me, let him follow me; and where I am, there shall also my servant be…”  John 12:26
we are placed in that position by reason of the new birth, born from above- seated with Christ!
You aren’t serving him if you aren’t following him- copying and imitating him!
Fellowshipping with the word has been demystified by the word himself- it is simply following and imitating the word as dear children, realizing we can because ‘now are we the sons of God!’
“So be imitators of God, as his dear children.”  Ephesians 5:1, ISV