You will need...
A top or sweatshirt (believe it or not!)
10mm jump rings x 50
4mm eyelets x 100
Eyelet punch tool
Marker or tailor's chalk
Patternmaster or graded setsquare
Can't think of anything too demanding here but getting everything perfectly neat and professional-looking can be challenging.
This took a lot longer than expected! Unfortunately, it took me somewhere between five and eight hours, although a lot of that was down to trial and error, so I'd say the actual work itself, without setbacks, would take somewhere within the region of five hours.
Turn the sweatshirt inside-out and cover the front and back with interfacing.
Decide where you want the slits to go. I set mine out at 10cm intervals across the front, using my patternmaster to measure parallel lines 6cm from each side of the top, followed by two lines for the 'edge allowance' (you'll see what I mean later on) - each 2cm apart - and repeating the process until I reached the centre. I repeated this process across the back.
Cut strips of Bondaweb to cover the edge allowance on both sides. Again, mine were 4cm wide and I wouldn't recommend making them any narrower, otherwise the eyelets may be too wide for them. Iron the Bondaweb in place.
Cut lines along the centre of the edge allowance (you should have these already marked out) with V-shaped tips at either end.
Fold the edges back on themselves, including the pointed tips at the ends, and iron them to fix them in place.
Turn the sweatshirt right-side out and punch eyelet holes, measuring them as you go along to make sure they're the same distance apart and that they line up. I set mine out at 10cm intervals. Insert eyelets. You will need to use the eyelet puncher for this part.
Finally, join the eyelets together along the slits by inserting the jump rings. After that, your sweatshirt should hopefully look like this: