I struggled with this so much. There was just so much I read that said it wasn't possible. But it works just like every other manifestation, with beliefs, expectation and longing all getting in the way. If I care about the relationship that much the only thing that's ever worked for me is letting it go. But the mindset 'if I let it go they'll come back' never works. Really, truly honestly letting it go. The time varies between a few days to a few weeks and they're back in my life. This works with any kind of relationship.

Yes, it's possible. Kind of. Like Bluebell said, you have to let go completely, which is hard. I went through this process with my now-boyfriend (3 years in and still gloriously happy together!).

I posted the full story here but the gist of it is:

1. I had the biggest crush I've ever had on someone who was under no circumstances interested in a romantic relationship (in general, not just with me). I didn't think he was my perfect match, I actually thought we'd be incompatible.
2. We started "casually" seeing each other - friends with benefits type relationship.
3. Even though it was hard, I made an effort to appreciate the relationship for what it was, without needing it to turn into a committed romantic relationship.
4. I kept visualising my ideal relationship without needing it to be him (as mentioned, I had doubts that he could fill this role).
5. When my feelings for him became too intense, I broke it off, but I kept getting signs from the Universe that this wasn't over. Finally I begrudgingly asked him out (sure that I was going to be rejected), and he said yes.
6. Turns out he was my perfect match after all!

The link I posted also contrasts this with a story I had earlier in my LOA days when I was trying really hard to manifest a relationship with a specific guy. He had even been open to dating me, but once I started trying, his interest dwindled (even though my actions in the physical world were the same as before).

I think manifesting a relationship with a specific person is one of the hardest things, because there are so many emotions tied into it. The way to go is to keep focusing on the vision of your dream partner, and simultaneously let go of needing your person of interest to fill this role. Just enjoy your relationship with them for what it is, and shower it with appreciation. Then, if they are capable of being your dream partner, it will happen.

Also, you asked about the timeline. I started crushing on him one year before the relationship. We became friends with benefits 6 months before the relationship. I had a lot of inner work to do with relationships and men in general, so I spent those 6 months cleaning that up as well. I actually had a breakthrough in healing a deep childhood wound related to romantic relationships, about one week before we started going out. So you want to make sure you do work in this area as well, and clear any negative beliefs or wounds you may hold.