Really the only way to keep them from curling is to dry them in a leaf or flower press. A simpler way is between newspaper sheets with a book holding it flat. I don't know any other way to keep them from curling up. Once dried you can mount them pinned in a glass case if you want. For something like hemlock with small needles that fall off, you almost need to embed them in clear resin or they fall apart.
"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky