Hi Dessiet and welcome to GOY :o)
I recently gravelled a large area of my lawn so hopefully my advice will help....You need to take the top layer of grass off first (not always necessary but it'll be better in the long run) put lots of good weedkiller down and cover the area with some weed fabric. Then just add your gravel and it's job done! Hope that helps? You'll probably get another response suggesting a different method, but this certainly worked for me. Happy Gardening :o)

You may probably need some sort of edging to help retain the gravel.

Agree Dido, I forgot to mention that bit :o)

I dont think this will last BB? I have done this in a very shaded side garden. I removed the top soil and dug out all the weed roots, I put railway sleepers as edging (for the look) but rocks, brick edging or treated wood would work as well. I put down 3" sand and leveled and firmed it, I the covered with 2 layers of weed suppressant membrane and 4" of 1/2 inch gravel and I rake it over twice a year. I did this 9 years ago and although mine is planted with Junipers its remained weed free and it has remained firm and level.

I've done this method in lots of areas now and it has lasted, the oldest area being 4 years old now. As long as the jobs done properly then I don't see why it wouldn't last many years :o)