When we introduced our Border Collie b!tch to our second dog, we took her to the place the second dog was. We rehomed our second dog (Millie) so when it was time to take go visit her we took our Border Collie (Tess) to the home Millie was in. We allowed them to approach and sniff at each other- when we felt comfortable with the introduction and felt that getting Millie would be great we took her home.
At first, Tess didn't want to associate with Millie when in her own territory and wouldn't even be in the same room as her. We left them to it and didn't force them on each other. Whenever Tess came in the room Millie was in she got praised, or whenever she played with her she got praised. We took then on walks together to build pack mentality and after around 2 weeks Tess came around. Now? they're the best of friends.
The best thing to do is take your Collie to meet the second dog on neutral ground...ground that she doesn't think is "hers". Don't force them on each other, and don't force your Collie to do something she doesn't want to do. If your comfortable with the second dog and do take her home just leave them to it, don't confine them to a room together and don't force your current dog to play, etc and take them on walks together.