At this stage, you should have the general shape of your vehicle. You should have a modified Volume, Area and Linear values. Now it's time to put the hull together.
There are two primary external features you will have to worry about: Endostructure and Armour.
From this point forward, everything added to your vehicle will accumulate both mass and volume. Your mass was chosen right at the beginning; your goal should be to use up every gram of that mass. You don't need to worry about filling all your volume, but if you want to be super-efficient, and you have lots of volume left over, you can go back and fiddle with it.
Mass is generally given in kilograms or in tonnes, and will be indicated as such. For those from the United States, there are 1,000 kilograms in a tonne. Remember to convert as needed.
Volume will either be given in liters or in cubic meters. There are 1,000 liters in a cubic meter, so again, remember to convert.
Cost is given in imaginary currency values, represented by the symbol ₠. One ₠ is about enough to buy a loaf of bread.