So first of all, whenever you need to know how to get a certain egg move on a certain Pokemon, you should just look the Pokemon up on Serebii.net and scroll down to their egg moves. For Farfetch'd, scroll down to First Impression under Egg Moves and click "details", it will link you right to Farfetch'd's details on how to obtain the move. For Farfetch'd, she can learn it through breeding with a Smeargle who knows First Impression, as they are both Field Egg types.
- Get a male Smeragle to learn First Impression, either from a Golisopod or another Farfetch'd/Smeargle.
- Get a female Farfetch'd and breed the two.
And you get Farfetch'd with First Impression AND NOW you know how to get any egg move you want!