You will need:
For my t-shirt I will be using a Metallica Death Magnetic t-shirt that is a men's size small.
1.The first thing you will need to do is to take your tape measure, and measure how deep you want the shoulders of the tank to be. I did mine at around 2.5 to 3 inches each side, if you want the shoulder width to be symmetrical then it is best to use a tape measure or ruler, if not then you don't need to bother. Mark this point in pencil on each shoulder. (TIP: make sure to measure from the neck outwards because that's the material that will make the shoulders).
2. Now begin cutting off the sleeve of the t-shirt, starting where you marked in pencil. I prefer to cut the sleeves off in a curved shape, so that the tank arm holes curve in slightly, but you could cut straight downwards if that's how you wanted yours. I cut my arm hole so that it finished in the same place as the original sleeve's armpit, but tanks also look really cool with a dropped arm hole, which is where the arm hole goes quite low (I'm going to cut the next one I do like that).
You should finish with something that looks like that! See how the arm holes curve in like I was saying before?
If you're happy with your t-shirt then you're done! There are further ways that you can customise your tank, you could shorten the length by cutting material away from the bottom to make a cropped tank (I didn't do this because the name of the band etc was at the bottom of my tee, which is why mine looks a little long!)
You could also customise the neck of your tank, which I decided to do because the original neckline on mine came up a little higher than I would have liked.
3. I decided to just cut off the "collar" part of the tank, to do this I made incisions down the collar and stopped just before reaching the seams. I did this about 4 or 5 times at various points around the collar, as you may be able to see from the picture below, although it isn't that clear..
4. Cut the collar off, keeping as close to the seams as possible (it's best not to cut the actual seams because it can make the neckline look a little untidy.
So you should end up with something that looks similar to this! You can obviously customise yours in your own way to suit how you want it, you could cut your arm holes to drop lower, the shoulders to be thinner or wider, the neckline lower or higher, I could go on! This is just how I do mine and a tutorial for anyone to follow if they need help starting off with their own D.I.Y tank projects! Hope this was useful and I can't wait to wear mine when I watch Metallica at Download Festival in a couple of weeks!!