Posted: 14/08/2015 at 12:33
AJP - this is probably not going to be a quick job.
1. Your first step should be to remove as much of the rubble and other rubbish from the top 18" of soil as you possibly can.
2. While you are doing that you should remove as many weeds (with their roots) as you can. The only way to do both these jobs is by digging with a fork.
3. When it's been dug once - leave it for a couple of weeks. New weeds will almost certainly appear - either dig them out or apply weedkiller to the leaves. You should use a glyphosphate based weedkiller to kill the weeds and their roots. It takes time for the glyphosphate to work it's way through the plant so they may not appear to be dying for several days - sometimes a couple of weeks depending on the weed.
4. You will probably need to repeat step 3 at least once.
5. Once you are sure all the weeds & rubble have gone you can start mixing in your topsoil
6. Then you can start levelling / firming / raking. Repeat this 2 or 3 times
7. Then you can sow the grass seed ..
So - much as you said - but with more emphasis on spending several weeks clearing the site of rubble and weeds before you bring in top soil.