This is likely to be tough, for two reasons:
Students' previous experience in math has probably been exclusively devoted to carrying out algorithms.
Their initial attempts will probably end in failure and negative feedback.
I teach mainly physics, not math, but as an example along these lines, I assign freshman physics students this problem: given the radius $R$ of the earth and its period of rotation $T$, find the velocity of a point on the earth's surface lying at latitude $\theta$. Over and over again, students show up in my office hours wanting help on this problem, and they usually have not attempted a sketch. If I then ask them to make a diagram without giving additional guidance, they usually draw something in perspective. I then have to suggest that they do non-perspective drawings either from the side or the top, or possibly a cross-sectional view. They find this utterly alien, and they also usually have not had real-life experiences that would help, such as taking a shop or architecture class. They require extensive guidance, with reminders that, e.g., a length has to be measured from a definite point to a definite point, and that an angle has to be defined between two lines that cross. Or they draw a triangle with vertices at the north pole, center of the earth, and a point on the equator, and label one of its angles $\theta$.
I think this is a case where something that you and I consider easy and natural (drawing a diagram) is actually an extremely sophisticated skill that needs to be built up over time. A similar example would be order-of-magnitude estimates, such as the volume of a cow or the number of piano tuners in New York.
Keep requiring them to do it.
Provide explicit guidance, possibly in the form of a list of guidelines. E.g., you can tell them not to make their diagrams tiny, or not to draw a diagram in such a way as to imply a symmetry that isn't known (e.g., drawing a triangle as isoceles when it's not known to be so).
Expecting them to do it without being forced may be unrealistic until they've had enough time to do it successfully a certain number of times and accept that it really is "normal" to do so.