That's geek talk for identifying the base shapes that your subject is comprised of and reproducing them virtually.
Keep in mind that the object that you are working on will be used to subtract material from a solid object... creating a cavity in which the real part will be placed.
Measure each dimension carefully.
Tinkercad is a great tool for the 'virtual re-construction' or modelling phase. Combine appropriate geometric objects along with your measurements in order to produce a digital / virtual positive.
If you need a little more detail here, give Tinkercad Quests a try.
Including using the new shiny MakerBot Digitizer. Which, might be overkill for this object... but it's something you should keep in mind.
I often print the model I'm working with and double-check its dimensions against the original. Measure twice.. cut.. er.
Okay, maybe that saying doesn't totally translate with 3D printing - but you get the idea.