How do you add infill to a self created 3d model?

I want to get into 3d modelling, I've been messing around with Sketchup and i have created some basic designs with it, I made a hollow test cube thinking i could use a slicing program to add infill to the empty void inside the cube. But this doesn't work. I'm obviously missing something. Can anyone give me a push in the right direction? Kar98k PUBG : https://downloadringtonesfree.mobi/kar98k-pubg/,

You need to make it a solid, and the slicer will create the infill with your slicer settings :)

Get the "Solid Inspector" PLugin. It will tell you what is manifold (solid) and what isn't.