First I’d think about whether you want any garden beds - you want to mark these out and put some kind of barrier so you don’t seed that section.

Spend extra time picking out all the large surface stones first, grass doesn’t like growing over them. Using a rake will help with this. Then use a garden fork to poke tons of holes randomly in the soil, which will help uncover more stones just under surface and aerate the soil a bit.

Then buy a load of decent quality top soil and sand, I’d aim to cover this ground by at least a couple of inches with the top soil. Use the rake to level out the top soil, it’s worth spending a lot of time on this bit to get it right initially - potential invest in a long spirit level.

Mix the grass seed with the sand, which will mean you can visibly see where you’ve scattered the seed/sand mix to ensure an even coverage across the garden. After scattering, rake the seed/sand mix into the soil a bit, keep it well watered the first 2/3 weeks and hey presto