It is seriously the easiest thing in the world. I kid you not.
Choose an HQ image and shrink it down to however small you want it. This was originally 1080p size, which I shrunk down to 1000px in width.
Duplicate the original layer and set it to COLOUR DODGE.
Invert the colour dodge layer and your image should disappear into almost all white (except for a few parts here and there). This is completely normal.
Now add a GRADIENT MAP layer in plain black to white (Layer -> New Adjustment Layer -> Gradient Map). Or you can also add a HUE/SATURATION adjustment layer and set the saturation to -100.
Click on the layer set to colour dodge and go to Filter -> Blur -> Gaussian Blur. Adjust the amount of blur according to your image. With this particular photo, my radius was set to 30.0 pixels.
Now the line art portion is coming into place, but we have all this background that we need to get rid of.
Click on Layer -> New Fill Layer -> Solid Colour and set it to white. Click on the layer mask portion and inverse it so that it’s black (you can use Ctrl+I if you’re PC or Command+I if you’re on a Mac).
Using the Eraser Tool, we are going to erase the background until it’s only our subject.
Tada! We are finished! To place it on a coloured background, just COPY MERGE (Edit -> Copy Merged) all the layers, paste on a new document with a coloured background and set it to MULTIPLY. Lower the opacity for a softer & more vintage feel.
That’s it! You are finished. Super easy.