First go at preserving deceased specimens..

[M Denae Etter]

How do you know when/if the specimen is dry?

It will be stiff and no longer flexible at all. Make sure you pinned it how you want it to stay during drying because if you try to move the legs after drying they will break off. I have overestimated whether it was dry and that is the worst. I was casting a female western black widow and her abdomen totally sunk in like a raisin afterward.