During Act 1 of Baldurs Gate you'll meet the Githyanki fighter Lae'zel. Recruiting her in the Nautiloid Crash Region should be easy by dealing with the tieflings who've captured her and than rescuing her.
After she joins your adventure she will be quite focused on finding her fellow Githyanki in hopes of finding help for your shared predicament, following her companion quest will eventually lead to your first meeting with Voss. If Lae'zel is with you during this encounter you can allow her to do the talking to skip some check rolls to convince Voss not to make you dragon food.
After the first time you meet Voss if you travel through the Mountain Pass you should end up in a new map, here you can find a Monastery by following the path winding downward. By entering through either the doors or underside cave entrance, you will find the Githyanki Crèche inside. Once here you will want to have Lae'zel with you and progress until the Githyanki Queen is asking you to destroy the presence within the artifact, you should agree to go inside but not kill your guardian as no matter what you do here you will need to fight your way out of the Crèche.
After this the next time you rest at camp you should have a scene where Voss will come to your camp and propose an alliance to take on the Githyanki queen, allow yourself and Lae'zel to accept and the achievement will be yours!
(If you're having an issue where the scene with Voss is not happening no matter how much you long rest try this: if you are in the room where the Queen tried to have you killed, on the left side complete the statue puzzle by having the statues face eachother to open the secret door. Walk through here until the red ring around your minimap dissappears, this means you can fast travel out. Travel to a waypoint in another area such as the Goblin Camp in the Nautiloid Crash Region. If working correctly you should instead end up in camp at nighttime and upon going to bed the scene should play)