As literacy curricula around the world expand to include multimodal meaning-making, the challenge that remains is how teachers can design engaging and effective learning experiences in this context and the nature of their guidance to students in developing their multimodal literacy. Our paper focuses on the topic of multimodal composing, where students create artefacts to learn and demonstrate their learning. We seek to understand how teachers can design for students' learning through multimodal composing with the use of a pedagogic metalanguage. Our data is drawn from a design-based research project on the teaching and learning of multimodal literacy in two secondary schools in Singapore. We discuss the implications of the design and evaluation of students' learning through multimodal composing and reflect on the nature of the design work by teachers as they negotiate the curriculum requirements and make sense of their professional learning.