Well I cut my own sides and back every week with a #5 guard. I cut the top of my sides and back with a #12 and I fade from #5 to #12 by hand. I cut my own hair for 10 years prior to wearing so I'm quite good at the fade. Then I use scissors to give the longer hair that meets the piece a bit of a choppy texture. I have the topper cut professionally though and I recommend you do the same. I have a very choppy cut which makes the piece much easier to blend because it breaks up any straight lines. Choppy hair is much easier to keep blended as it grows out and the difference in length isn't apparent.

I know I keep posting this pic but I believe it's relevant to this thread.