Pin clamps are used to hold balsa parts to the building board. Putting pins through thin strips of balsa weakens the wood.  Sometimes we use 1/2″ lengths of 1/16″ x 1/8″ balsa with a pin through it as a finger to hold the parts to the building board.  These often split, are too soft to hold or can be used only once.  Those shown here are made from pins and plastic disks punched out of HDPE plastic carton lids with a leather punch or paper punch.  I dump a few of these plastic circles on the building board and push pins through them while they are lying flat on the board. Some of the thicker ones grip tighter than the thinner ones and can dent the wood if pressed hard. The pins might bend in the thicker, harder plastic, hold the pin with two fingers and push it through with the dowel pin pusher.  Push them down just enough to hold the wood, not so hard that they dent the soft wood.  The same can happen with the wood blocks if pressed hard on soft wood. Put one pin on either side of the stick to keep it in place.  On the wings, the ribs will hold the spars from the inside.  On the fuselage sides, the uprights will hold the longerons against the pins placed around the outside.