Designers-by-assignment, or subject matter experts (SMEs) who are pressed into training service, have become common in the workplace. A review of more than 24 studies on expert and novice instructional designers, however, revealed that little is known about how designers-by-assignment think about design and make design decisions in the field. A qualitative study was therefore conducted to explore and describe how a sample of designers-by-assignment, expert instruction librarians, carried out design work in the field. Two research questions guided this study: How do community college instruction librarians design one-shot library workshop sessions? How do community college instruction librarians make decisions during the design process? The multiple-case study design included seven participants with at least 10 years of instructional experience in a California community college library environment. Qualitative data sources included a recruitment survey, semi-structured interviews, think-aloud protocols, and artifacts. Audio recordings of interviews and design tasks were transcribed and member-checked, then coded and triangulated. An event-state network and descriptive narrative were produced for each case. A final cross-case analysis of all cases included a composite event-state network, several matrices of design practice components by participant, and a composite descriptive narrative for the final workshop product and each research question. It was found that participants followed four process events (Inquiry, Planning, Implementation, and Evaluation) comprised of numerous design inputs. In addition, four contextual factors (Expertise, Theory, Culture, Institution) seemed to underpin the participants' design decision process. Although participants were successful in designing a workshop, their focus on content and implementation skewed their view of the instructional design process and its integrated nature. The contextual factors found in this study aligned participant design practice with an ill-structured problem-solving approach to instructional design rather than the well-structured ADDIE process. From these observations, the efficiency and effectiveness of librarian design practice could be improved by expanding the role of instructional designers to oversee the work of these designers-by-assignment and to create assistive design tools. Further research on the nature and needs of designers-by-assignment could ultimately lead to improvements in the quality of their instruction.