How to make a 3D digger cake [Excavator cake]

We call them diggers, some people call them excavators, Richard Scarry called them power shovels.  Whatever you call them, if you want to make a cake like the one below then you're at the right place.

Yellow: Doiles and Diggers Birthday Party

Ruby turned 3 recently and we had a Yellow-themed birthday party (it's her favourite colour).  Here are some of the details ... make sure you scroll down to the cake - it's worth it, trust me.

Mini cupcakes with washi tape flags, sitting on washi-taped cake stand:

Yellow paper straws, napkins, yellow crepe paper table runner, paper doilies painted yellow on the wall:

I don't mean to boast but there were also balloons.  Yellow ones.  I didn't want to generate any party envy so didn't bother with photos of the balloons.

Finally, the cake: