They are created with Crystals arranged in a closed shape, with a Book Receptacle placed anywhere along the outside area. Unlike Nether portals, book portals need not be any particular size or shape, as long as the crystal wall is completely closed and does not pass a chunk boundary.
The created gateway will open when a descriptive book or a linking book is placed in the receptacle, and will remain open until the book is removed. Two portals may not be activated using the same crystal block(s). It may be a bug, but usually when portals are connected by crystals (for example, having two across from each other with a crystal walkway between them) activating one will deactivate the other.
Players should be aware that unlike Nether portals, crystal portals do not create a return portal. A linking book or other means will be required in order to return.
Similarly to star fissures, entities can fall through the portal and appear on the other side. This can be used to move farms around, and allow players to move mobs between ages. Currently falling sand, gravel, and TNT have issues in single player, and do not appear on the other side as expected.