What is the best way to prepare groped to make new flower bed?

Groped? Lay down clear plastic that you purchase by the roll from the lumber yard. Weight it with rocks or sand. By the end of summer the plot will be ready for planting. In a hurry? You'll have to dig the sod up and discard. Till or spade in compost. You'll have more weeds to deal with than if you use the plastic method.

Remove turf and set aside either to re-use or stacked face on face to use later when rotted down for pot plants etc. Turn over the Soil. Add compost or Soil improver and fork in well. Now you are ready to plant......

See if this helps!

https://www.bhg.com/gardening/yard/garden-care/building-a-new-garden/

Removal of the sod turf would be the first suggestion. Some would say to do a soil test or you could possibly imagine your soil with compost. Potentially add a black plastic weed barrier put your plants in piercing the barrier and finally mulch.

First, you need to pull out all the grass and weeds. Turn the soil and add some compost or rotted manure. Turn again. I also add some topsoil. Plant your flowers and mulch around them to prevent weeds from growing and to hold moisture from watering.

remove every thing 'green' or that had ever been green down to about 6"; rake, make sure you have ALL roots, bulbs, etc., out or they WILL come back.... work in a load ++ of top soil/compost, and some people like the 'weed barrier' fabric, I don't; I prefer mulching around/between plants with layers of newspaper -- it will decompose, and you just keep adding more, this way you can change where/what you plant -- with the fabric weed barrier -- you can't dig thru it, it will get caught on rakes/hoes, etc. and pull up, etc.