In design process , a mockup is a model used for demonstration, design evaluation or promotion and it shows mainly feedback from users. This model is really usefull for representing our ideas, iterating and amending to improve it.
Designers use different kinds of tools to sketch their ideas. There are no best or worse tools, anything that can easily represent your inspiration and creativity, is usefull. For me, it usually starts with pen and paper. Sometimes you will find inspiration while being outside of your office. In that case it is awesome thing to have sketchbook at arms reach.
Sketch or just written idea – you never know when they may occur.
Following a brief from a client, you start producing a design. Some prefer to have several different mockup examples, some just one for improving it. I prefer using just one because if the design is inappropriate, you can start again without too much time wasted on other directions. Also, clients are more involved and it is easier for them to understand simplified mockups.
Mockup is an important part of design process. Putting ideas on a paper without analysing it too much is starting point. Next step is making mockup in preferable tool and producing design which you think it is the best for the client.