Kurmasana is an intense spinal forward bend and I recommend should only be practiced with a teacher and not by learning from the internet.
This post is mainly to show how long my spine is when I practice this posture rather than as a 'how to'. I have pictures and a video at the end.
Many people can do a version of this where they straighten their legs but their spine remains very humped up--like a tortoise shell I suppose.
It will indeed remain a little 'humpy' but my effort here is in trying to lengthen.
I do active variations first to firm the tummy (in a way I can breathe and move the spine). A more active variation would be to lie down and do what I show here.
First, a sort of navasana where knees bend and I make efforts to draw them into my chest.
Then, I try to keep my legs as close to my chest as possible and straighten them. This will be beyond a lot of people. Don't let your legs come out too far from your chest.
The video below shows the posture in action. I make particular efforts to also ensure my neck is in a comfortable position and you can see me moving it around.
Following an intense forward bend like this I do a less intense forward bend like paschimottansasana and then a strong tummy firming posture like bakasana or a high plank. If you do postures well counterpose is not necessary but I have just made it a practice.
I teach something like the 'wowee active hip opening' sequence I just posted prior to this before I do a posture like kurmasana.
This is the approach I take in my classes, workshops and retreats. Join me if you like--in Canberra, Bali (for our yoga and raw food retreat September 2015) or in Sri Lanka (yoga retreat). Details of my class and retreat schedule are on the home page of my blog.
Happy and safe practicing.