How do I sew a 3D curve?

[Bill]

3D? I can't think of a better or proper explanation. Anyhoo...
Here is my problem. I think I've pinned it right, but how do I sew it? Pretty please with loads of sugar on top.
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[Lachica]

Carefully! Ok, a bit flippant. You may need to clip the curved edge - ok, just enlarged it & I see you have. That's exactly what was needed.  If you're concerned about sewing over pins you could baste it first. If it's a firm fabric it should be easier than on a floppy fabric which will tend to stretch then pleat itself.

[Kad]

@Bill
This is similar to the situation when setting-in a sleeve.

A couple of suggestions:

Sew slowly

Try a mock up in scrap fabric, if you've some of similar stiffness so much the better but anything else will let you try your technique.

Make sure that the machine needle is set to the LHS, like you do when inserting a zip - this will help with keeping an accurate SA.

Clip into the seam allowance of the straight edge at several places along the length of the seam.

I would sew this with the curved piece on the top with the straight edge piece nearest the feed dogs, but that's just me and not a rule.

[BrendaP]

I agree with putting the straight edge to the bottom, it's better if you can see the curved one.  Then just sew slowly.  It's not generally recommended to sewover pins but if you are going slowly enough it won't hurt if the needle does touch a pin as it will just slide over it.

[Bill]

I never rarely leave pins in. I learnt that lesson early on.

[Ohsewsimple]

I never sew over pins.  The newer machines won’t just glide over but break a needle!   
Id be inclined to tack this first. 

[Esme866]

If you value your machine - no matter its age - pins should never be sewn over on purpose for any reason. Not just about breaking needles - (first one I ever broke as a kid landed in my eye)  - you can also screw up the timing on your machine- and other damage.

I would definitely hand baste this first. It looks as though you've clipped right up to the seam line, so I would baste just beyond the seam line (1/16" or less) not on the seam allowance side - unless the needle will leave noticeable holes. Pin properly at the apex and each end and one additional pin in between and then hand baste. You will be able to control the fabric  easily to ease the fullness into the straight piece while you're working by hand. Then remove all of the straight pins and preferably sew with the curved side up so you can watch for creasing.

I'd make the basting stitches no more than  1/4" long. The smaller the basting stitch the less likely to develope creases.

Take your time and you'll do fine. :thumbsup:

[Ploshkin]

All of the above good advice plus - just a few stitches at a time, stop with needle down and adjust fabric each time.  If it's only one or two stitches at a time it's progress.  It can help to have a tool (small scissors, stitch ripper, awl, point turner) to hold the fabric in position as it goes under the foot so that the excess doesn't end up pleated as the foot goes over it.

[Stitchalot]

I would suggest pinning in the direction of the stitching, along the seamline (remove as you stitch) - putting them at right angles holds the fabric too far in. Better still, baste very close to where the seam will be. Also, using only a small seam allowance will help. Stitch with the larger side downwards - slowly! Good luck.

[Bill]

Thank you all for you help 
I did it, sort of.
Before I'd read all replies I unpinned all the pins and just sewed each short section a bit at a time, repositioning the fabric as I went along using a zipper foot to get as close to the seam as I could. It kinda worked.
Whether it was my calculations or just plain dumba55ness I'm not sure but the corners where the 3 pieces join didn't match up very well.
Frustration has made me scrap it and go with another design. It was going to be a wash bag for Pam for when we go camping. A dinky rucksack thing is now in the works.
I shall attempt 3D curves again at some point.
Once again, thank you all 

[WildAtlanticWay]

Perhaps it’s just me, but when I’m trying to sew a tricky seam on something that isn’t clothing, I tend to use a narrow 4mm (?) double sided tape to stick it together before sewing.

I’m not sure what the stuff I have is called, but it doesn’t gum up the needle.

[Ohsewsimple]

Wash away basting tape @WildAtlanticWay   
Great stuff!