The first time I cut a tenon I started by marking it out with a marking gauge, then with 
complete confidence I made my cuts and removed the waste material and after a little clean up I test fit the tenon into the mortise only to discover the shoulders of my tenon were out of square, forcing me to fuse around with my shoulder plane until I finally achieved a good square fit. This problem stayed with me for sometime, I would cut a tenon and then have a problem with the shoulder not being square. I tried using different marking gauges thinking maybe this was the problem,  but nothing seemed to make a difference. Eventually I realized the problem was never in the marking gauges, but in the material. When using a marking gauge to mark tenon shoulders the gauge references the end of the board, and if the end of the board is not true it will cause the shoulder line to be out square, and it doesn't take much to have an issue. But if you reference from the side of a board these marks will be accurate. So what I started doing is using my starrett and a marking knife to lay out my tenon shoulders referencing from the side of the board. Since I started created tenons this way I no longer have fit issues with my mortise and tenon joints.  

​​One of the most common questions is how do I build this thing ? This table, box, chair or whatever this thing might be. Honestly just build it. The bottom line is the knowledge of how to build something comes from building it and the more you build it the better you will be at it.  Nothing is a better teacher than repeating the same task over and over again, especially with the basics of woodworking.
Over thinking it will always put a fast ending to what would have been a great idea. As woodworkers we always push our selves to try and be better than the last thing we created and we should, because this is how we grow both in knowledge and ability. But as a person building something for the very first time ever, it can be intimidating when you look at the way advanced woodworkers do things. Mortises, tenons, dovetails and even half blind dove tail, it is a lot to understand when you have no knowledge of any of it.  My best advise to the first time woodworker is to just do what works for you, then do it again and again. This is how you gain the knowledge your looking for and grow in this craft. Because there is no rule that says you have to build anything a certain way.