well.. if its going through your opponent then i think you were probably out of range. Its a pretty big hitbox and shouldnt be too difficult to connect with. As far as te SHFFLing goes, im not sure how familiar you are with L cancelling, but you have to do that. Press the shield button within 7 frames (thats 7/60 of a second) of hitting the ground to cut your landing lag in half. As far as getting punished with fair goes, you are going to be punished a lot by using it unless you space it on shield (which it is actually very good at doing). technically, based on the shieldstun data, GW can SHFFL a fair onto a shield, and have enough time to do a spot dodge, up b, or dash attack and avoid getting shield grabbed. however, that assumes you landed right after the attack connected, and did a perfect SHFFL, which is very hard to do well with GWs fair (since it comes from above, lots of hitlag, and his fast fall speed isnt that great).
my advice, use bair, dair, and uair for shield pressure. They have landing hitboxes, and you can connect with an up b right afterwards to keep yourself safe/start combos. you can space a fair on shield, but if you get to close, your are going to get shield grabbed unless you are facing ZSS or you are m2k, the later of which is extremely unlikely. you can find videos of how to SHFFL on youtube, i would recommend just going into training mode and practicing your L canceling.