Look into "Brew-in-a bag" aka "BIAB". Deathbrewer has a great tutorial stickied here somewhere, easy to find.
How much grain you mash depends on your equipment - how big your brewpots are (I found it best to use one pot for the mash, a second pot for the dunking).
You should make friends with a brewing calculator - there's only about a dozen or so online, for free or for a nominal cost. Beersmith, Brewtarget, Tastybrew, Hopville, Brewmaster's Warehouse, etc etc etc. Learn how to swap out amounts of base grains and extracts and get the gravity you want. Once you can do that, ANY recipe will be within your reach.
Cheers!