At the end of the day (century) it should be as straightforward as making beer, an espresso shot, bread or using a fertilizer. The end-products and techniques are quite simple, even ancient.

However the search-space to get to those solutions is enormous, and getting to those very simple conditions is still a significant (if no longer herculean) undertaking. If you're given the correct tools and a good protocol it's really pretty easy to accomplish useful and interesting biological tasks (again you already do this when you make beer/bread/yoghurt/kimchi). But for the time being engineering an entirely novel biological function would require a fairly sophisticated understanding of the context you're trying to fiddle with.