Before i mastered the art of making a cutoff shirt that is even on both sides and doesn't look like a chop job, i searched the internet forever and never found a help for making this type of shirt. It isn't a "tank top" it is more of a lat cutoff which has deeper arm holes and is overall more comfortable and free moving shirt for exercise. I have done this over and over again with no problems while making the shirt. The amount of material removed will expose your side from your shoulders down to just above your hip as you can see in the end picture.
Step 1: Step 1: Materials and Tools
Only need two things,
1. SHARP pair of heavy duty scissors
2. the shirt you want to convert
(optional) chalk for marking
I highly recommend the bigger pair of sharper pair of scissors rather than the generic kindergarten safety scissors strictly because the bigger ones cut faster and don't hack up the shirt and make a cleaner finished shirt as well as making time shorter. You will want to do this on a hard surface such as a table. If you are nervous or a first timer and don't want to mess up a shirt with a logo on it, practice on an old shirt that you wouldn't wear or a cheap white t shirt a couple times so you grasp the concept, it's really easy.
Step 2: Step 2: Making Lineup Marks
Now for the fitting, put on the shirt, go to just above your hip on your most outward part, approximately 2 inches above your waistline, and grab a little bit of fabric. This will be the end of the cut, take a thumb and finger grip of the fabric then take the scissors and make a snip as a reference mark (make sure its big enough so you can see this and DON'T cut yourself, bring the fabric away from your body). Next, go about 1-2 inches inside the top of our shoulder (or wherever you want the cut to end) and make another reference mark (again, thumb and finger grip, pull away from skin so you don't cut yourself). If you really want to be dead on or want to make sure you follow the right line when cutting, use the chalk to make guidelines for the scissors, i personally don't do this but if you think it will help you go ahead, the chalk washes right off.
Step 3: Step 3: First Cut
Now that you've made the reference marks we can begin the cut for the first side. Take the shirt off and lay it flat on the table(front facing up), the key here is to make sure the shirt is evenly flat on both sides, make sure the sleeves are flat and the collar is even and line the bottom hem of the shirt up with itself. Also lineup the top seem of the shirt(the one that goes across the shoulders) is flat with the table, in other words make it so the seem is visible but adjacent to the table. I can't stress this enough, the shirt must be completely flat otherwise the whole shirt will be off and not be satisfied with the end result. Start at the bottom of the shirt at the reference line and start cutting, don't make a straight line from bottom to top, all that's needed is a simple half circle cut(see picture) that progressively cuts towards the top reference cut. Try to make as few cuts as possible to avoid the raveling of the shirt, but use the sharper end of the scissors for best results. Your first cut should look similar to the above picture.
Step 4: Step 4: Second Cut
Next is the most important part of the whole process, it is essential that you make sure the shirt is lined up correctly. To do this, you fold the shirt over lengthwise(arm to arm). Line up the collar as shown and make sure the whole collar is flat and even. make sure the inside and outside halves of the torso are together and lay them on top of the uncut fabric. Then line up the bottom hem on top of each other and make sure they are flat and even then adjust the torso again to make sure the whole shirt is flat. Once you've made the lineup, make the first cut, snipping from the top(collar) of the shirt downward (for some reason it is more uniform this way rather than starting from the bottom). When cutting, you will want to tilt your hand slightly to the outside(away) from the shirt so you can get the material evenly below the previous cut lines, only a SLIGHT tilt (5 degrees or so), you'll know if you're doing it right because the cut will line up perfectly. End result should look as above.
Step 5: Step 5: Finished Product
And you're done, the shirt should look as above, both side holes should be even in width and length due to the fold-over cutting. Feel free to experiment and make the shirt thinner or wider depending on your taste or if there are logos on the shirt. Have fun with it!