Sound morphing stands out among the sound transformation techniques in the literature due to its creative and research potential. The aim of sound morphing is to gradually blur the categorical distinction between the source and target sounds by blending sensory attributes. As such, the focus and ultimate challenge of most sound morphing techniques is to interpolate across dimensions of timbre perception to achieve the desired result. There are several sound morphing proposals in the literature with few open-source implementations freely available, making it difficult to reproduce the results, compare models, or simply use them in other applications such as music composition, sound design, and timbre research. This work describes how to morph musical instrument sounds with the sinusoidal model using the sound morphing toolbox (SMT), a freely available and open-source piece of software. The text describes the audio processing steps required to morph sounds with the SMT using a step-by-step example to illustrate the need for and the result of each step. The SMT contains implementations of a sound morphing algorithm in MATLAB ® that were designed to be as easy as possible to understand and use, giving the user control over the result and full customization.