In one aspect, a fixturing member for fixing a source of flowable material relative to a pen injector so as to permit introduction of the flowable material into the pen injector is provided herein. The fixturing member includes a body having a web with opposing proximal and distal faces. A distal wall extends distally from the distal face of the web, with the distal wall at least partially encompassing a distal chamber. Features are formed on the body for removable mounting onto a pen injector. Also, features are formed on the body for mounting onto a source of flowable material, A cannula extends through the web the cannula having proximal and distal ends with a lumen extending therebetween. The distal end of the cannula is located in the distal chamber and positioned such that, with the member being mounted to a pen injector, the distal end is located to be in the pen injector. The proximal end of the cannula is located proximally of the proximally face of the web such that, with the member being mounted to a source of flowable material, the proximal end of the cannula is located to be in communication with the flowable material. Advantageously, with the subject invention, a fixturing member is provided which facilitates mixing of substances in a pen injector in preparing a pen injector for injection.