You can follow a video (Blender 2.80 and later) of this guide. It may be helpful to refer to both the video and this text.
You can open this start file for Blender 2.80 and later (may work with 2.79 and earlier, but has not been tested) and skip this section Follow these steps to prepare for previewing liveries
Blender Render to Cycles Render in the dropdown to the right of the topmost menu bar.Object Mode and select Material+ NewMaterial EditorNode EditorPrincipled BSDF or Diffuse BSDF with its BSDF output connected to a Material Output by its Surface input. If so, skip the next three steps. A then delete if this does not clear the Node Editor)Add > Output > Material Output and click anywhere in the Node Editor to place the nodeAdd > Shader > Diffuse BSDF and click anywhere in the Node Editor to the left of the Material Output node to place the nodeBSDF output of the Diffuse BSDF node to the Surface input of the Material Output NodeAdd > Color > MixRGB and click in an empty space to the left of the BSDF node to place itAdd > Texture > Image Texture and click in an empty space to the left of the Mix node to place itColor output of the Image Texture to the Color 2 input of the MixAlpha output of the Image Texture to the Fac input of the Mix nodeNode Editor, click the icon and select 3D ViewA to select all (press until orange outlines are seen)Delete and click on the confirmation popupFile » Save, or File > Save Startup File as this is a good starting point for previewing liveriesFile > Import > Wavefront (.obj)Import OBJMaterial tab on the Properties panel, click the ball icon next to + New and select the material created earlier. If you are using the start file linked above, view uv.000 or view composite is good for most livery making. Base Color [Mix]Fac [Image Texture]OpenOpen ImageInstead of repeatedly opening the livery texture file every time you make a change, you can install a plugin that will automatically reload it every time the file changes.

File > User Preferences…Add-ons then Install Add-on from File… at the topInstall Add-on from File…user under Categories on the leftSave user settingsAuto-Reload ImagesTimer button (with a clock icon) in the Context tab of the Properties panel