On both an increment core and a cut face, wetting the surface may make rings more visible. Also, shaving a thin layer of wood from the surface will often make rings more visible. If you are felling the tree, use a sharp handsaw to make sure the rings are clear.
On the increment core (or stem cross-section), locate the pith. Generally, the pith is an oval-shaped area of darker, spongier material. Confirm that youve got the pith by making sure that the rings closest to it form concentric circles around the pith.
To locate the first ring, consider where the core or cut was taken within the internode. At the top of the internode, little wood has formed so the first ring is very close to the pith. At the bottom of the internode, more wood has formed so the first ring is farther away from the pith. Also, consider the thickness of wood on the leaders of the sample trees. A species with a thick woody leader (such as Pl) will have the first ring well away from the pith.