Dan is giving good advice with the help page where he is pointing. It is important to know where you are going to draw the lines. The coordinate I used where based on the extent to the featureclass. Enter the properties van the shore lines and look at the Source tab:
It seem to be cut with vertical lines at 156°E (+156) and 180°E (+180). This will be different with each tile of the data. To give some handy tips on constructing the lines I will include the procedure which isn't complicated.
In ArcMap right click on the layer, and select Edit Features, Start Editing
In the standard toolbar there is button to show the Editor toolbar:
...which will show this Editor toolbar. Click on the Create Feature window
In the create features window click on "cd06e" and choose Line in the lower part of the window:
Click on F6 to enter exact coordinate:
Specify the exact coordinates (start point) of the line you are going to draw. For instance Long 180 and Lat 0. Click Enter to confirm the location, which will draw the first point. Hit F6 again and specify the end point of the line:
Enter to confirm the second point and press F2 to end the line.
Repeat the process for the second line, and save your edits:
... and stop editing.
That's all it takes.