Following the instructions below, it's possible to create a centered stripe, running from the front edge of the body all the way to the rear edge.

First select "Decal" from the Livery Editor menu screen.

　

Select "Body" as the area to edit, then select "Add Layer" from the right-hand side of the screen. If you're unsure of what parts are included in the area you're currently editing, then press R3 to highlight the area light blue.

　

For the camera option, select "Roof".

Select a decal from the selection of presets under "Shape 1". Find a preset that conforms to the stripe you want to create.

　

The decal will be provisionally placed on the roof of the car. By default the decal will be white, which may make it difficult to see. In such a case, exit the decal editing screen and adjust the color under "Decal Color", then return to the decal editing screen.

　

Using the L1 button and the right stick, stretch the rectangle horizontally (towards the rear end). You can rotate the decal with the L2 and R2 buttons. At the same time, also adjust the width.

　

Continue stretching the decal until it reaches the edge of the trunk. Exit editing mode and select "Projection Range". Adjust and set the "Limit Angle" and "Limit Depth" to a high value until you can see the decal reach around the front bumper. After you've finished your adjustments, press "OK". Once again select "Edit Decal" to re-enter editing mode. Using the R1 button and right stick, rotate the camera to check how the decal looks.

　

So now the body stripe is complete. Next we'll create a stripe for the hood. This time select "Body 2" from the "Editing Area" menu.

　

Select a decal from the selection of presets under "Shape 1". Find a preset that is similar to the one used for the body.

　

The decal will be provisionally placed on the hood. Using the width of the decals on the front bumper and roof as a reference, modify the hood decal until it is the same width. Using the R1 button and right stick, rotate the camera to check how the decal looks. Zooming in can also help for checking accuracy.

You can also select "Direct Input" to adjust the magnification ratio (vertical/horizontal), degree of rotation and opacity.
(Some values cannot be adjusted for decals that have modification limit.)

　

After you've finished applying a stripe to the car, use "Decal Color" to paint the selected decal whatever color you want. Your stripe is now complete.