IMHO, you can best start by creating a (base) model, then add mates and finally add dimensions.

Pick you x/y/z starting point with care, for every part or model you create, it makes life a lot easier when assemblies are made.

Another tip in creating mates: try to make use of the Front-, Top- and Right-planes as reference for your mates and dimensions, if possible in any way. Clever thinking and planning ahead helps you save time an frustrations afterwards.

I recently had to work with someone else's models in where tons of construction lines where created to define the outlines and these are a real pain when any modification is needed.