When I make interlined curtains I use a side turning of 6cm and a single hem of 12 cm.
This produces an uneven mitre at the bottom corners, which can be difficult to fold in order that the mitre meets tidily at it's inside corner.
I needed to find an easy method of producing this mitre quickly - for myself when making curtains as commissions, and for my soft furnishing students, who needed to remember the method in order
to make a mitred corner when not in class.
So, for years now I have been using and teaching the following method that I 'invented' in my workshop.
Having pressed up a 12cm hem and a 6cm side turning, treating the main fabric and interlining as one,
I mark the outside corner with a pin - just taking a tiny amount of fabric - holding the corner down onto my padded work surface.
I then use a pin, just to mark the hem where it meets the side turning at a right angle and another to mark the side turning where it meets the hem.
Open up the turnings and then fold the corner in - in a line from the pin in the hem,
to the pin in the corner, and to the pin in the side turning. Press well.
Open the corner so that it is flat again, and trim the interlining off through the crease line and discard.
Refold the corner over the cut edge of the interlining, making sure the corner pin is still in place.
Refold the side turning and hem. Stitch a covered lead weight into the corner -
just catching it to the flap of main fabric inside the mitre.
I make a string of weight bags from curtain lining before I start any new project.
I usually make more than I need so that I always have some to hand.
Once the mitre has been refolded neatly,
give it a gentle press with the iron.
It should stay in place , but may need pinning if the fabrics are springy and keep unfolding.
Finally, ladder stitch the two folded edges of the mitre together, starting at the outside corner
and working towards the inside corner.
By gently pulling the ladder stitching tightly,
taking care not to pucker the fabric,
the folds will come together closely and neatly.