I have a client that wants this exact cake for this weekend. I'm thinking of just using the end of one of my modeling tools to create the lines in the dress, but I'm checking to see if anyone else has a better suggestion. I'm the worst at getting straight, even lines.

photo?  link?

Dang it! I copied and pasted it and I guess it didn't work. Let me know if  you can't see this.

Absolutely love that cake!

I'd probably use a ruler to lightly mark the lines before using the modeling tool to score the lines.  I guess you could lightly mark the lines with a ribbon cutter also, but you'd have to be careful not to mark them too deeply.  Sorry I couldn't be of more help, but would like to know how it goes.

I would probably try rolling the fondant out over something like corrugated cardboard to get the lines.  You'd obviously need it to be something that the fondant wouldn't stick to, though.

Thanks for the suggestions. I will probably end up using the end of the modeling tool to get that deep indentation on the dress and TAKE MY TIME.  You're right about the ribbon cutter as it would possibly mark it too deeply. Of course, I'll post the finished product! :)

How about marking the lines with a dowel? You can just push it into the fondant and it will leave a straight groove. Probably quicker than using a pointy tool and a ruler. 

I would mark the lines with a cooling rack!  The bars on mine are about that width apart, and you wouldn't have to individually score each line

ADoing the lines with a dowel is a good idea. I may have to reinforce it with the modeling tool but that'll keep it consistent.