Step one is turning on the Gameboy, and adjusting the contrast to see just how many display pixels are missing.
Step two is to remove the back cover of the DMG, depending when it was made, this is where you'll need the Triwing screwdriver. There are six screws, two of which are under the battery compartment.
Once you take those screws out, don’t pull the Gameboy apart quickly. There’s a ribbon connector connecting the screen of the Gameboy to the circuitry on the reverse of the console.
For the sake of this fix, we'll be leaving the ribbon attached to the reverse of the gameboy, you'll see why soon.
Next, you have to remove ten screws holding the display board onto the front case. From this point forward, all the screws we’ll be dealing with are Phillips cross heads.
Remove the screws, and gently wiggle the front board away from the case. Don’t be too violent here, the board is very thin, you could put some stress fractures in it if you’re not overly careful.
Eventually the board will come free. If you’re having some real difficulty, double check that you have removed all screws, and it isn't being held down by corrosion.
It’s a good idea at this point to place a slip of paper between the top board and bottom board so that you don’t short anything out you don’t want to.
Once you’ve done this, put the batteries in the Gameboy, turn it on, and change the contrast with the knob on the left side of the unit until the whole thing turns black. Bar the damaged pixels.
At this point, your gameboy screen may resemble something similar to the photo above.
Just below the display there’s a thin black rubber strip attached to the brown circuit ribbon. Carefully remove it, and the adhesive. So it looks like the photo above. From here, it's nearly ready.
Heat up your soldering iron and gently rub it back and forth on the brown part of the connector just below the screen. Depending on the heat, you may not even need to make contact with the iron.
The brown ribbon connector is kapton film, which is incredibly heat resistant, so don’t worry about heating it too much. Just don't stray onto anything else.
While you’re doing this you may notice that more lines appear while you’re heating the connector. This is just some product of the solder reflow process.
Once you stop the heat and the solder solidifies, the screen should steadily come back to normal if done correctly.
If fixing the lines is all you wanted to do, you’re done! Go ahead and reassemble your Gameboy, and don’t forget to reattach that rubber strip under the display, this protects it!