The same basic tips apply for Merrill. Be mindful of her when you talk to NPC’s, finish all companion quests and find the character specific gifts and present them.
Merrill and Fenris are bad choices to have together if you’re intent on wooing her, as Fenris wants nothing to do with magic. It is best to keep Varric and Isabela in the party when doing her quests throughout acts 2 and 3, as they are the least disturbed by her actions and Blood Magic.

Merrill joins the party when you complete ‘Long Way Home’.Be polite at all times and don’t frown on her blood magic use. Be mindful, however, of the fact that choices made in conversation with Merrill can change your relationship with other characters. Complete ‘Welcome Home’ once she joins.

‘Mirror Image’ become available once you’ve completed ‘Blackpowder Courtesy’. Talk to Merrill in her home and agree to help her for 15 friendship points.

After the varterral has been defeated during ‘Mirror Image’, don’t choose “Don’t waste your tears.” for 5 friendship points. Then, give her the arulin’holm for 15 friendship points at the end of the quest.

Visit her in her home and flirt. If ‘Questioning Beliefs’ is available by now, complete it. Head back to your mansion at night and Merrill will come by. Ask her to live with you.

Pick up Merrill’s first gift at Master Ilen’s Shop in Sundermount. Give her the gift at her home for 15 friendship points.

Her next gift is the Sylvanwood Ring, found on the thief leader’s corpse during ‘A Murder Of Crows’, gotten by speaking to Nunico in Hightown.

Make sure to finish ‘A Talking To’, ‘On The Loose’ and go see Merrill at her old house to unlock ‘A New Path’, her final companion quest.

Of course, support her decisions and stay on her side throughout this quest. Comfort her at the end.

Finally, talk to Merrill at the Gallows Prison or in the Gallows Courtyard (depending on your end of game decision) during the “The Last Straw.” This will officially complete the romance.