Please keep in mind I'm in no way a professional seamstress, I just kind of figure stuff out as I go and what's the best way for me may not be the best way for you to do something. Feel free to share if you have feedback/construction criticism.
In case you're sewing for your first time: unless the instructions say otherwise (like on the sleeves) always turn everything inside out before sewing it (unless you like the unfinished seams on purpose thing that's been pretty trendy recently) and don't forget to sew back and forth a couple times any time you start and finish sewing, that way your stitched won't unravel.
You want to start with making your pattern pieces. The easiest thing to do is a trace a shirt you already have that fits well. Keep in mind the measurements I listed are just what fit me well and won't necessarily be the same for you. P.S. They're in inches, but I'm sure you figured that out-
If you like a normal tshirt type neck and your design in high enough you can line your pattern up with the very top of your shirt and not cut the neck hole at all. My design was too low for that and I didn't plan on keeping the neckline like that anyway.
It's easiest for me to cut the front of the shirt off, line up my pattern with the design and then cut it out (that way I don't accidentally make any cuts on the back part).
I didn't include seam allowance on my patterns so I made sure to leave about 1/4 of an inch all the way around the pattern.
Front piece ready to go:
Sleeves are tricky. If you like yours puffier then you should make your pattern longer, if you like your sleeve to go farther down your arm then you need to make the pattern wider.
Fold pattern in half if you're going to use original sleeve or leave pattern open if you'll be cutting it out from a flat piece of fabric (say extra fabric from the bottom of your shirt, if your original sleeves were not 'tall' enough to use)
Now for the ruffles at the bottom you're going to want to cut out 3 to 4 long strips of two inch wide fabric from the extra fabric at the bottom of your original shirt.
Yay, now you have all your pieces and you're ready to start sewing. I used a regular sewing machine for all of this but there is no reason you couldn't use a serger (would probably be even better) or hand sew it (except I have no patience for hand sewing).
I start by sewing up both sides, then the shoulders and then carefully sew the neckline area. Be careful to not accidentally sew closed your arm holes or neck holes. If you want to do a regular style neck hole check out option 2 down at the bottom of this post. When I make girlier styled shirts I like to just fold over the edge of the neckline sew it down.
Sewing down the edge of the back of neck hole:
You can also use two "front" pattern pieces if you'd rather have the neckhole be rounded all around, mine is rounded in the front, straight in the back.
When you sew on your sleeves your shirt will be inside out and you'll tuck the sleeve into the shirt but the sleeve part with need to be right side out. If you're not sure you have it right- pin it into place all around, flip the shirt right side out and make sure you don't have any seams showing.
Xray vision= It looks like this but remember that the sleeve should be inside your shirt at this point.
But when you sew it, it will look like this (keep in mind, sleeve should not actually be on the outside of your shirt at this point)-
I sew the bottom part straight and then some largish pleats in as a sew the top part together. You put tiny pleats all around if you'd prefer. To be exact, I sewed up the front part of the sleeve 5 inches straight, added two pleats and then went back and did the same to the back part of the sleeve. That way I can just an an extra pleat if I guestimated the sizes wrong.
Pleats at the top of the sleeve:
When you flip everything inside out it should look like a big fluttery sleeve at this point:
Time to make it a puff sleeve. You'll need some narrow elastic:
To figure out exactly how much- measure around your arm where the sleeve would be. Mine was about 10 inches long.
I mark the middle of my elastic, that way while I'm sewing I know if I get to the halfway point of my elastic and I'm not yet at the halfway point of my sleeve that I wasn't bunching the fabric enough as I went.
You can either sew a channel at the end of your sleeve and then feed your elastic through, or what I did was sewed the edge of the sleeve right onto the elastic. Place your elastic far enough from the edge of your sleeve that you'll be able to fold the rest over and cover all of the elastic. While you sew you need to punch/squish your fabric AND pull the elastic tight (which helps it bunch/ruffle up even more).
When you're done the inside of the sleeve should look like this-
and the outside of the sleeve should look like this-
Almost done- ruffle time! You'll want to take your strips of fabric and sew them together so you have one super long 2 inch tall strip of fabric. Fold over the edge and sew that down (unless you like the unfinished look, since it's tshirt material it's not like it will fray)-
To make the ruffles/pleats I just folded over the fabric a bit every inch or so.
Flip your ruffles down so they're now faces the right side up and you're all done, although you might want to iron the ruffles, it helps them lie flatter.
Neckline Option 2
If you like a regular tshirt collar but weren't able to keep the existing one because of the placement of the design then you can just sew it back on.
Cut the collar off the original shirt.
Your tshirt should be right side out at this point, your collar should be upside down and inside out. I only pinned it halfway around because I knew I wasn't going to keep the collar on, you'll want to pin it down all the way around.
This is what it would look like when you're done-