Top down simply means that you start to model in the top assembly and then spawn components as the design requires.
There probably isn't that much by way of tutorials available, but that is partly because it is just so darned easy to work in top down assemblies. You simply model some or all of your new component in the top assembly and then you use the Assemblies tools to create a new component. The dialogs offer prompts to select entities assign a filename and then delete from the top assembly everything used exclusively for the new component.
By the assemblies toolbar there is an icon available for the create new component function.
When creating new components you generally need only select the solid that you wish to create as a separate component. The system is capable of identifying the associated objects such as any sketches, sheets points curves and datums that you may require. Depending settings that you choose in the dialogs these entities may be deleted from the top assembly and created separately in the component files, however not all construction geometry is always captured for deletion from the top assembly so an occasional manual cleanup may be required.
After you create the new component you'll need to save the top assembly once before the component file is written to disk.
If you use mating conditions then it is in the nature of things that they can only be applied after the component has been created. In the same way the inverse is true of linked geometry in that shared relationships between objects crossing the boundary between assembly and component. If you require them then their use will not be meaningful until you have created the component, and as a logical part of how you create your design you may wish to make decisions about how you want to work things like wave links before you create the whole design. My advice is to avoid this to begin with as I realise that anyone starting out may not easily comprehend such comments.
You can also work bottom up which means creating all components in separate files before adding them to the assembly. There is however no firm rule stating that one has to do either and most designers use a combination of both techniques.
What makes life even easier is that you can work in context by making a component the work part while the assembly at any level is still the displayed part.
If you're still in doubt by all means post your queries. It sounds as if you may just be hesitant to make a false start, but most people once they get started working with assemblies find they're able to fill in those gaps fairly rapidly.
