Making flat pieces such as lamellae or scales is easy. Because it is hard to predict the exact amount of shrinkage, you may want to first to water harden the whole piece of leather, then flatten it under a cutting board or something similar for a few minutes, then cut out your lamellae with a utility knife and add holes with a leather punch; at this stage in the process the leather can still be cut fairly easily.
If I want my lamellae slightly curved in the horizontal direction, to provide a little additional stiffness and to better fit my body, I take a cooking pot with a diameter of a foot or more, line up the lamellae around it while they are still flexible, tie them on with a strip of rag, and let them dry that way.
Making a vambrace, a rerebrace, or any other piece that is curved but not stretched--any shape you could make from a sheet of paper--is equally easy. Cut the piece of leather a little bigger than you think you need (remembering to allow for shrinkage), harden it. Wrap a towel around your forearm to protect it from heat and provide extra thickness to allow for padding (or wear your gambeson, if it covers your arm). Tie the piece of leather around your arm with strips of cloth (string will leave marks on the leather). Leave it there for fifteen minutes or so. Untie it and take it off, being careful to keep the shape--at this point the leather is still fairly flexible. Trim off any surplus. Leave it somewhere to dry. Instant vambrace.