i wouldn't worry about the different temps and holding it for twenty minutes at 120 then raising the temp to 150... i would advise against adding direct heat to the grains in the mash tun if you are using a big pot on a stove. i always ended up burning some grain no matter how attentive i was to stirring it.... rather if you are gonna to temp steps, have more hot water ready to add to the mash to raise temp. that bible book has a good mash schedule for increasing temp with infusions of hot water.
how i would do it...
i would take my five quarts of h20, bring it up to 170 degrees, remove it from the heat or put it in your tun, depending on your set up, then put the grain in ontop of the water, give it a whirl and put the lid on it. the grain will bring the temp down to 155 or so and you can begin conversion. it should be good to go in an hour. begin your sparge... you do not want to throw away any of the liquor you generate from the sparge. the first half gallon may be cloudy and murky, you can pour it back into the grain bed if you wish, or you can just go with it, the turbid runnoff will settle out with the hops when you cool your wort.
the partial mash will give you a feel for working with the grain and getting your sparge method down. it will add an increase in flavor and freshness that extract can't, but remember the bulk of your fermentables will come from the dried extract you are gonna boil.
i've done about 35 gallons of partial mash brewing and i really like it, now i am assembling my all grain stuff for this summer