If you mean you want a flowerbed where the lawn is or in it, mark out the shape and size of the bed you'd like with string, then start digging up the lawn.
The grass you dig up/have can be piled up (green side down) and eventually it will rot, though it may take some time.
Hi Planando,
I don't know where you are, but here in the US and also in the UK you can rent a sod stripper if your yard is large.
Be sure and add about 3" or 4" of compost to the flower bed and mix it in before planting. Your flowers will thank you.
Newt