We start out with a simple 4 layer background scene. A gradient sky and three planes of rolling hills with a few trees. The Inkscape file to this tutorial can be found here. When creating your own scene make sure you change the colours slightly to make the planes stand out and be distinguishable. When exporting to png making sure you have the transparency turned on.
You can create the blur right in Inkscape by using ‘Filters/ blur’ and then e.g. a motion blur. In the filter properties you can set the effect parameters. Setting them both to the same value turns the motion blur into a even blur.
Usually, I personally prefer the ‘cleaner’ approach and keep the vectors non-blurred. The effects are added in a bitmap program for better control.
Open Gimp or the bitmap editor of your choice (as long as it allows layers and blurring) to stack up the bitmap layers.