The Store module must be in use before the Assemblies module can be implemented.
Before entering an assembly you should enter the details of all of the components required using the Store module's partnumber tool.
You will also need to determine how you are going to calculate the overhead and labour costs for the assembly. Your accountant should be able to help you decide this, perhaps by calculating hourly rates for these costs. Or you could ignore them. The costs - not your accountant.
Start by choosing an assembly that is fairly typical of the products your company produces. Enter the assembly using the assembly tool.
Roll-up the cost of the assembly using the roll up assembly costs option, and check that it is correct.
Having satisfied yourself that you are entering assemblies correctly continue to add the assemblies for all of your products.