One of the first ways that I brewed all grain was with the Brew in a Bag system.  This system allows a extract brewer to advance into all grain without having to buy to much more equipment.    The critical part of BIAB is having a large enough pot to hold your entire grain bill and the full boil volume of water plus the water that will be absorbed by the grain.   Another factor that makes it a very simple method is that it uses a direct fire method for maintaining mash temperature.   The BIAB method also minimizes the transfer of wort from your mash tun to the boil pot.  Of course this method is not without issues, but has been proven to make some very fine beers.