But first, I'd just like to point out the obvious fact that this process is totally optional. Ready-to-wear clothing is sold all the time with unmatched stripes. If the stripes on your dress don't join at the seams, the world isn't going to end. If anyone points out that the stripes on your top are a bit wonky, well, they're probably not the kind of people you want to be hanging out with. If you just wanna get on and sew without taking these additional steps, go for it - life's too short!
Still here? Okay, so on to the tips. My stripe matching strategy begins before I've even cut the fabric...
1) Laying out fabric
When you fold your fabric in half and lay it out ready to cut, you want to make sure that the stripes on the top layer are exactly on top of the stripes on the bottom layer. Insert a pin along the top edge of one stripe, then turn the fabric over and check that, on the bottom layer of fabric, the pin is positioned along the top edge of the same stripe. If it's not, reposition it. Do this on a few stripes, every few inches or so.
2) Laying out the bodice pattern
If you're taking the time to match stripes, a key area to pay attention to is the side seams. When you pin the front bodice pattern piece to the fabric, line up the bottom corner of the side seam of your size line with the top or bottom of a stripe. (If you look closely, you can see the bottom corner of size 3 is on the bottom of a stripe - if you can't see the stripes through your pattern, you could trace the pattern onto semi-translucent paper.) Also take note of whereabouts in the stripe sequence the notch and the top corner of the side seam land. Now when it comes to pinning the back bodice pattern piece to the fabric, line up the bottom corner, notch and top corner with similar points in the stripe sequence.
Transfer the pattern outline and markings to the fabric - I like to use dressmaker's carbon and a tracing wheel, which will allow you to remove the pattern pieces before cutting out and therefore get more accurately cut pieces, which is extra important for stripes. Don't cut your fabric just yet though...
3) Laying out the sleeve pattern
The curved shape of the sleeve head means that the stripes probably aren't going to match up along the whole armhole seam. (Although I did manage to match them all up on my Nautical Knit Dress, but to be honest I can't remember how I did that and it probably won't happen again!) If you're being diligent though, you can match up at least one sleeve stripe with one armhole stripe. I would go for the one in the middle when you're looking at the finished front bodice front on, as it's probably the one that's most visible when you wear the garment. You can see in the above photo that that's the only stripe I've matched on the armhole of this Coco top (and, even then, the diagonal angle of the sleeve seam means that the sleeve stripe overshoots the bodice stripe by a mm of so).
On your front bodice piece of fabric, find the armhole notch, then move your eye 15mm (5/8in) inside the raw edge to where the stitching line will be - take a mental note of the position of this point in relation to the nearest stripe (ie. is it on the top edge of a stripe, 5mm away...?). Now when it comes to laying out the sleeve pattern piece on the fabric, position it so that the point 15mm (5/8in) within the raw edge of the front notch of the sleeve (the single notch) is at the same point in the stripe sequence. This isn't an exact science because of the diagonal angle of the sleeve seam, but it works for me.
Oh, and also double check that the bottom edge of the sleeve is exactly parallel to a stripe so the stripes match up at the underarm seam. They should do if the fabric is on grain, but knit fabric can get a bit twisted sometimes. Again, transfer the pattern shape to your fabric but don't cut it just yet...
4) Before you cut the fabric...
Once you've transferred your pattern outline and markings to the fabric and removed the pattern pieces, double check that the stripes on the bottom layer of fabric are still sitting exactly underneath the stripes on the top layer. Insert some pins through a few stripes and check they emerge at the same position on the underside of the folded fabric, just like you did when laying out the fabric.
Okay, enough faffing, now you can cut it out!
So your pieces of fabric are cut out and ready to sew. You may now want to pin the front and back bodice pieces together with the pins parallel to the side seams so you can try the garment on and fit it to your body. Once you're happy with the fit (and have either marked on the stitching line or trimmed the raw edges to 15mm away from the stitching line), remove the parallel pins. Now pin the side seams together again, this time inserting the pins perpendicular to the seams, along the top of each stripe (or every other stripe, or every few stripes, depending on how far apart they are). Just like you did earlier, check the pins are coming out in the same place in the stripe sequence on both the front and back bodice. Make sure the pins are crossing the stitching line, ie. 15mm (5/8in) within the raw edges.
Whether you're planning to sew the garment together on a regular sewing machine or an overlocker (serger), it's a good idea to baste (tack) first on the areas where you want the stripes to match. Thread your sewing machine in a contrast colour and set the stitch length to 4mm. If you've got a walking foot or dual feed foot for your sewing machine, attach that as it will help feed the layers of fabric through the machine evenly without one of them shifting along a bit faster than the other. Baste stitch just within the seam allowance - ie. about 12mm from the raw edges. On the armhole, you could just baste the one pair of stripes you want to match up.
Once you're happy with the basting, rethread your machine and reset the stitch length - or get out your overlocker - and sew the seams for real. Now you can pull out your baste stitching with a seam ripper.
If you are the type to bother matching stripes, do you have any tips I haven't mentioned? Do share!