Romance Guide for Mass Effect helps the players develop a romantic relationship with their desired companion. You will get a chance to interact with them, creating a deeper and meaningful relationship that can even lead to sex. Completing the romance subplot, awards you the Paramour Achievement.
Romanceable characters in Mass Effect
Liara can be romanced no matter what gender you have chosen
After completing Eden Prime, and finishing speaking with Captain Anderson, you will find either Kaidan Alenko or Ashley Williams, (it depends on Commander Shepard's sex), feeling guilty about what happened during ending of the Mission Prologue: Find the Beacon. If you state that there was no way to know how the beacon would react. This will make Alenko or Williams feel better about themselves and is a good starting point for a possible relationship.
First Main Mission
After finishing the Mission Find Liara T'Soni, you can start courting her after she admits that she's been looking into your background. You will have another encounter with her, after you find the second beacon.
If you are already involved with Kaidan or Ashley, both love interests will confront you after the debriefing, asking for you to choose between one of them. Choosing one, automatically ends the other romance option.
During the Mission Virmire: Assault, Captain Kirrahe will ask for one of your squadmates to join the diversion team, while you and your team infiltrate Saren's base. You can select between Aidan Kalenko or Ashley Williams. One will go with Captain Kirrahe and the other will follow you. Sadly, at the end of the Mission you have to choose who you save.
If you have continued the romance plot up until the end, while you are traveling to Ilos, your love interest will visit your personal chambers on the Normandy. Paragon choices will net you a passionate intimate cut-scene. Taking the Renegade option makes the love interest leave the room. This doesn't mean that the romance ends, you can continue it on the next game regardless of your choice here.
Citadel: Asari Consort
If you complete Citadel: Asari Consort, an Assignment that takes you to stop a Turian named Septimus to stop spreading rumors about Sha'ira and proving her innocente to Xeltan. Sha'ira will offer a reward for you, but if you are visibly dissatisfied, she will initiate a sexual encounter. This won't grant you the Paramour Achievemnt