The following works well on all grass types I've encountered in the eastern non-deep-South parts of the country, and I suspect on most other grasses too. Only caveat is that it assumes you have 10" or so of topsoil or topsoil-and-decent-subsoil... if your topsoil is much shallower, you can still do it but will need much more compost (etc) and much more mulch on top:
Step 1. optional but very strongly recommended: Do not plant the whole 20x50 area this year! Too much to dig over and much, and also too much to do first-year weeding all at once. On the part you don't plant, lay down corrugated cardboard topped with lotsa mulch hay etc, or cover with black plastic. Pull rogue weeds promptly and fix the holes they came thru. Then next year you will have an easy (tho not weed-seed-free) time of planting that area.
Step 2: Dig the whole garden over, going as deep as you can (full depth of shovel blade) each shovelful, turning each shovelful upside-down as you replace it into the ground, so that when you are done, the grass is upside-down and buried underneath the dirt as much as possible. Note that you will have to remove some initial shovelfuls to get room to put subsequent ones in upside-down - dirt always takes up more room after digging
Chop the grass and roots out of these 'spares', then throw them back into the garden. Leave the shoveled colds of dirt loose and lumpy, do not try to break them up or smooth them out.
Step 3: put mulchy composty stuff on the garden, like coop cleanings, mulch hay, well-dried weeds, untreated grass clippings if you have any, compost, etc. Distribute them evenly, not in piles. Leave the garden alone all winter.
Step 4: In spring when ground is workable, lightly fork in that mulchy top layer so it's somewhat mixed in and while you do that also break up any remaining clods and rake smooth. I recommend you mulch the garden immediately after this, even if you're not planting yet -- but the way you've prepared the garden you do NOT need insane amounts of mulch, a couple inches will do it.
A few weeds or spindly grass stems may still poke thru, but very few - pull them promptly.
Thereafter, the less you disturb the soil the fewer weeds you will have (all soil has a nearly-infinite supply of weed seeds lurking, awaiting a shot of sunlight to let them sprout!). Do NOT till or dig over the bed unless special circumstances require, and try to weed or hoe as gently as you can. It will make your life much easier !
Good luck,
Pat
Step 1. optional but very strongly recommended: Do not plant the whole 20x50 area this year! Too much to dig over and much, and also too much to do first-year weeding all at once. On the part you don't plant, lay down corrugated cardboard topped with lotsa mulch hay etc, or cover with black plastic. Pull rogue weeds promptly and fix the holes they came thru. Then next year you will have an easy (tho not weed-seed-free) time of planting that area.
Step 2: Dig the whole garden over, going as deep as you can (full depth of shovel blade) each shovelful, turning each shovelful upside-down as you replace it into the ground, so that when you are done, the grass is upside-down and buried underneath the dirt as much as possible. Note that you will have to remove some initial shovelfuls to get room to put subsequent ones in upside-down - dirt always takes up more room after digging

Step 3: put mulchy composty stuff on the garden, like coop cleanings, mulch hay, well-dried weeds, untreated grass clippings if you have any, compost, etc. Distribute them evenly, not in piles. Leave the garden alone all winter.
Step 4: In spring when ground is workable, lightly fork in that mulchy top layer so it's somewhat mixed in and while you do that also break up any remaining clods and rake smooth. I recommend you mulch the garden immediately after this, even if you're not planting yet -- but the way you've prepared the garden you do NOT need insane amounts of mulch, a couple inches will do it.
A few weeds or spindly grass stems may still poke thru, but very few - pull them promptly.
Thereafter, the less you disturb the soil the fewer weeds you will have (all soil has a nearly-infinite supply of weed seeds lurking, awaiting a shot of sunlight to let them sprout!). Do NOT till or dig over the bed unless special circumstances require, and try to weed or hoe as gently as you can. It will make your life much easier !
Good luck,
Pat