By using the consist editor.
Open the MSTS editors and tools, and select the activity editor. Then go to file and select "New". Then look at the bar on the right hand side of the screen, the central section of which is headed "Player". Under this is a drop down box labelled "Player servce". Click the "New" button underneath this. This will then bring up a dialouge box entitled "Service editor". In the middle of this are two-drop down menus; the top one is labelled "Consist". There are four buttons underneath, labelled "New, "Edit", "Use as template", and "Delete". Click "new" to make a brand-new consist. To edit an existing consist, selcet it from the drop-down menu and click "Edit"-but be careful, as if this consist is used by an activity, modifying it could throw the activity out of balance. To create a new consist based on an old one, click "Use as template"-this allows you to make a modified version of the consist without over-writing the old version. The "Delete" button is self-explanatory-but be careful, as if you delete a consist required by an activty it will crash MSTS.
The editor itself is fairly easy to use, you should work it our pretty quickly.
Another helpful hint for the future-use the search button!
