About stock modular base building: Back in September 2013, forum user Temstar posted a thread called The art of modular base building, about how to move modules around and align docking ports on different modules. They used these concepts to create the Modular Base Creation Kit, which is made for building large bases whose modules easily connect with each other (all stock, but there are modules made for kethane). It was made with munar bases in mind, but the concept can be used for any body with a solid surface. Both threads have very interesting pictures showing what can be done with this concept. But for those that dont want to use the stock docking system for making modular bases, Kerbal Attachment System has always a very popular solution to that because of its pipe system, which allows you to use a kerbal on EVA to make a pipe between 2 vessels, which functions very similar to placing struts in the VAB/SPH. The pipe merges the vessels, just like when docking, allowing transfer of resources between them using the right-click menu. However, some people think that this is too cheaty and/or not as fun as the stock docking system, because it allows you to just place some landers together, connect them all with pipes, and call it a base. But using features from KAS and it's sister mod Kerbal Inventory System, you can also bring parts with you stored in containers and assemble them in the field to make a base (or whatever you want to build), as if you were building on the VAB/SPH.