I built my first frame in January/February of 2013. It was completed just before I went to the North American Handmade Bicycle Show (NAHBS), and gave me a real appreciation for the amazing craftsmanship on display at NAHBS.
Things I did right:
I studied the Paterek Manual. This book provided a great overview of building a frame.
I bought bikecad. This greatly simplified designing the frame, and made mitering tubes a breeze.
I built this frame off of a granite surface plate, which really works pretty well for the occasional frame. I just realized the picture below if of an aborted mountain bike frame, but the technique is the same.
I ended up with a frame that I rode for a year and a half.
Things I did wrong:
I bought my flux and brass rod from the local welding store. I came to learn later that the flux and rod geared towards bikes is of higher quality and flows better. Moreover, using silver to braze instead of brass works much better, especially for a newbie. The silver flows really well, and allows for lower temperatures. It costs substantially more, but is well worth the cost.
The lugs I used were not formed well. I decided to use them anyway, but in retrospect, I should have obtained different lugs. The slightly misshapen lugs caused problems with alignment and flowing brass.
I really overcooked my joints. As a result, I had to ream the heck out of the seat tube so I could slide a seat post in.
Some practice joints might have been a good idea too.
I spray painted the frame. I realized all of the flaws on this frame, and didn’t want to spend the money on a proper finish. I knew I would build something much better in short order.
I made the front end too tall. I think I was feeling old and decided I wanted a higher front end, but I went overboard and it gave the bike a somewhat vague feeling up front.