it's actually very simple ... BUT, when you say "from scratch", to me that means a completely stripped down pan (IOW down to bare iron again with no traces of the previous seasoning on the pan) ... below is a pic of what ALL my iron looks like before i ever start a seasoning cycle on it -- complete, raw, stripped cast iron ... it's battleship grey when you get it to this point ...

once stripped, the process is a simple 3 step process - 1) warm the iron up to 200 deg in your oven; when it hits 200, pull out the pan & wipe on A THIN COAT OF CRISCO & THEN (seems counter-intuitive but trust me) wipe it off with a lint-free dry cloth to wipe the EXCESS Crisco off the pan; put it back in your oven 2) raise the temp to 300 deg & punch 20 minutes on a timer - after 20 minutes, pull the iron out & wipe the excess Crisco w/the lint-free dry cloth & put it back in the oven, 3) raise the temp to 475 deg & set a timer for 1 hr 15 mins & let it bake on the first layer of seasoning; when this timer goes off, turn the oven off & let the iron completely cool down INSIDE THE OVEN (which can take a few hours or so) ...

for my USER iron, i do this 3 step process once & then start cooking on it; for my SALES iron, i'll do this process another time or two b/c well seasoned iron looks good in pictures ... the pan is non-stick after the 1st seasoning cycle, however ...