Select a mission in the left window. Whilst holding the CTRL key, right click in the grey box in the right window. The mission will appear in the box. Add a new row and put a mission in it the same way. Now, in the right window, drag and drop the mission from the first grey box onto the mission in the second grey box to link them together. Note a sexp is created for the branch from the first mission to the second in the bottom left sexp window. Add a repeat link to the first mission so if they fail they have to play again.
Once you can do the above you can work out everything else.