there are too many steps in that..
1. The first border --- Image>Canvas Size -- click the Relative box and put 2px in for height and width, you'll see at the bottom of the dialogue box there's a box at the bottom for Canvas Extension Color, Select Black, hit ok
2. The second border --- Image>Canvas Size -- put 6px in both boxes and choose White for the Canvas Extension Colour, hit ok
3. The third border --- Image>Canvas Size -- put 50px in both boxes and choose Black for the Canvas Extension Colour, hit ok
Additional - before you do the above
1. resize image from high res -- File>Automate>Fit Image.. put 742 in both height and width (742, as alot of sites prefer 800px as the largest pixel size, if you choose 800, then the time you've added the border you'll be @ 850px)
2. Sharpening -- Filter>Sharpen>Sharpen, then
3. Edit>Fade Sharpen (60%, mode luminosity)