Conventional modeling practices allow users to create a part and combine multiple parts in an assembly. It is worth knowing that this is not the only approach to modeling. You have the option to model straight from assembly mode.
Consider a new assembly:
From this new assembly select Insert > Component > New Part. A new generic part file will be created in the feature manager design tree:
Select your part, right click and select “Edit Part”
In this example let’s sketch a simple rectangle on the top plane
Extrude the rectangle to create a solid. Please note that we are still in Assembly mode, However, we are working under the “Edit Component” feature.
Similarly we can insert a new part and sketch on a rectangle on the right plane as shown. Please note that the collinear relation was used to define the rectangle.
Extrude to create your second part
We can repeat the process to insert a new part and sketch on the front plane as shown below to make the third part
With all three parts complete, you may apply your materials to the parts, in this case various woods were selected.
Remember to save your new assembly with the new parts created from the assembly. In the saving process you will encounter this message indicating that you will need to save the parts that were created. Select save externally in order to have the part files created.
Finally, you will find your part files created in the same file location as the assembly.