Only thing that ended up working for DD in the long term.
We started at 7mths, and it worked wonderful. The first few nights were really hard and heartbreaking, but as each night progressed it got easier.
We would do the bedtime routine, put her in her cot, give her her blankie, say night night, then walk out, if she cried we would go in, comfort her without picking her up, then walk out and slowly would make the gap longer and longer. If she got distressed we would get her up and try 10 mins later.

We did go backwards a number of times and have to do it all over again, but that was only after some big milestones and she went back to being a great sleeper after a night or two of doing the CC routine again.