I’d recommend watching this video first (How To: Sewing Invisible Zipper by Made to Sew) to get a rough idea of how to sew concealed zippers:
The photos in this tutorial are from the second t shirt I modified and I made some changes as learnt a few lessons with the first one. Firstly, I would buy the proper branded concealed zipper foot for your machine. I bought a cheap one to begin with and had a lot of issues with the grooves not being deep enough so the zip kept jumping out of place. This is the Singer one:
Secondly, I’d recommend using YKK zips at least 41cm as they are better quality and it gives you a decent length to open the t-shirt properly.
Finally, if you can then use iron on stretch interfacing to stabilise the t-shirt where you will be cutting and sewing. It will make your life so much easier.
Pop the t-shirt on and mark where you want the zips to start. I set mine to the side of the bust area rather than across the middle of the boob for comfort and they’re more hidden that way.
Use a ruler to draw a line where you want the zip to sit on the front of the t-shirt, marking the top and bottom of the zip length clearly so you have a start and stop mark.
Mark two parallel lines ¼ inch either side of that line. Unlike my photo below extend these two lines to the top and bottom of the shirt to aid with the finishing later on.
Cut two lengths of the interfacing that will cover the full length of the t shirt and go slightly either side of the outer two lines you have marked. Follow the instructions to attach this to the inside of the t shirt, covering where your marks are on the front (usually just press on with hot iron).
Once cooled cut carefully along the middle line to just above and below your start and stop marks.
Now we move on to attaching the zip so at this point definitely have a look at the youtube video above as it’s really important you sew the zip the right way. Lay the zip flat on the t shirt and undo it. You will take the left-hand side and fold it out like a book (so hold the teeth that are pointing inwards and lift up and out so they now point outwards).
Holding the zip in that position place the notch at the top that stops the zip coming off in line with the start mark on the left hand line you have drawn. It is really important you get this right on the mark to ensure the two sides of the zip line up and close properly.
Pin/clip the full length of the zip in place with the teeth on the line.
Using the concealed zipper foot line up the teeth in the left-hand groove. Your needle should sit just to the right of the teeth as you want to sew as close to the teeth as you can to hide the zip.
Sew the zip in place slowly using your fingertips to hold the teeth slightly out of the way as shown in the youtube video as you do not want to sew over the teeth (trust me, I did this on my first one and had to unpick it all which took forever!). You’ll have to stop sewing when the foot hits the zip head at the bottom. Trim your ends really close as you don’t want loose threads getting caught in the zip when using the top.
At this point you will have one side attached, flip it back over to the right so the zip sits right side up and try doing the zip up. If it won’t do up you have sewn over the teeth somewhere so you’ll have to unpick and start again.
Repeat for the other side of the zip, ensuring you pin it the right way and use the right-hand groove of the concealed zipper foot this time.
When you approach the bottom of this side you may find the bottom of the zip gets in your way. You can flip it out of the way by tucking it under itself, just remember to flip it back once finished or you’ll end up thinking you’ve sewn it wrong as it won’t do up (the video covers this in more detail).
Tuck the top and bottom ends of the zip inside the t shirt and double check your zip works ok. It should now look like this:
You’ll have gaps at the top and bottom of the zip that are not sewn which we will now finish off.
Fold your t shirt so the two outer lines you marked are lined up and touching each other and pin in place (right sides together) pulling the zip out of the way.
Swap to the normal zipper foot your machine should have come with and starting an inch or so below the bottom of the zip sew back towards the zip as close as you can to the lines so that your stitches will meet the stitches from sewing the zip in. (This bit is hard to explain, watch the video to see how it is done).
Then flip your t shirt the other way and using your normal sewing foot sew remaining length of the outer two lines to continue the seam allowance down to the bottom of the shirt. Below photos should make it clearer.
From the outside your t shirt should now look like it has an extra seam sewn down the front to the bottom hem.
Repeat at the top, you may find undoing the zip a little gives you more room to work.
The only difference this time is rather than continuing the same seam allowance to the top seam I sewed it more like a dart so faded it out just before you hit the top shoulder seam.
The top hole should now be closed up too.
Repeat for the other side of the t shirt to put the 2nd zip in and you’ll then be left with something that looks like this:
Note how this t-shirt is also bump friendly 😉
All done! Hopefully that made some sort of sense and you’ve been able to produce a working t shirt. You can open each zip as much or as little as you need to nurse comfortably. Enjoy.