Implementing Level-1 MATLAB S-function

I'm using a Level-1 MATLAB S-function in simulink. Its

[sys,x0,str,ts] = fcn1(t,x,u,flag)

• u's size is 2
• x's size is 3

1. How should I use this in Simulink/MATLAB?
2. Should I enetr t and flag? How?
3. How should I deal with inputs and outputs? Using DEMUX and MUX? How?
4. What's the priority of u,x.t?
5. What're the outputs that I receive?

I'll appreciate if you give me an full example.