One approach is to envision the shape in 3D, typically with the aid of additional information. In another thread Kyle said
Kyle may have started with an initial vision of what the rotary tool should look like based by a physical example, then created the sketch based on that vision. When he did the 3D modeling he presumably had the sketch to work from but also his vision of what the object should look like in 3D.
Also in these videos Kyle is generally creating a 3D concept model with few constraints.
Look at simple objects around you and think about how they may have been designed. Learn to initially ignore fillets and other transitions and concentrate on larger surfaces. I frequently try to guess how the object was originally designed and/or made.
Kitchen utensils and tools can be good examples. Pick something you have which is simple and small enough to set by your computer. Then try modeling it. Initially don’t worry about exactly matching dimensions or shape. Instead concentrate on how to structure the model to represent the object.