How do I prepare a old/new plane iron

I was given a stanley hand plane so I've watched loads of youtube videos on how to get the plane ready for use but when I tried lapping the back and sharpening the blade I couldn't get a sharp edge so I've bought a new plane iron to try again. When I lap the back what grit sandpaper should I start with as I've seen some use as low as 80 grit but other people have said they wouldn't go any lower than 400 grit. The problem I see is when you watch the youtube videos they only use a plane iron or chisel that they wouldn't use in a shop so it doesn't matter if they used 80 grit sandpaper.

 

I've already watched the video you posted and also his chisel sharpening video . When he lapped the back of the chisel and did the bevel he said that still doesn't feel sharp to me then he went and used his diamond stones. I used the same grit wet and dry as paul sellers and I couldn't get a sharp plane iron. I've read on forums where people have only gone to 1200 grit wet and dry and they have had a sharp chisel and plane blades.

 

Are Stanley planes that bad or that I bought a new plane iron.

 

Chris tribe used 80 grit to lap a new chisel. I used 80 grit on a old chisel and it put a deep scratch on the chisel due to the grit not all the same size.

 

What grit should I start at. I think I started at 180 grit.

 

When I was given the plane it wouldn't cut butter and it looks like it's only been sharpened to 25 degrees. I don't know when the bevel would need to be reground to 25 degrees so maybe I should of done this first as I don't know how many times the blade was sharpened. What diamond sharpener do you use because I got a 3 eze lap diamond stones but I find the super fine (1200 grit) is to course and leaves lines in the bevel.