First thing is to create a very accurate plane body. If a great deal of care is taken the worst condition you typically get is a plane body that is slightly smaller at the top as compared to the dimension at the sole. Typically you can still make the infill dead square and given the shape of the infills the plane sides will expand just slightly as you slide them into place. This makes for a perfect fit and a rather square plane.
I use my milling machine to get my infills close to the final dimension and this also goes a long way to keeping them very square, however when it comes to the final fitting nothing works quite as well as a hand plane. A well set up plane with an extremely sharp iron allows you to work in increments not possible with any other tool.
I had fit the Gabon Ebony infills in the 875 plane pictured above. A weather system containing a lot of moisture and unseasonably high temperatures came thru our area. Two days later when I attempted to install the rear infill it would not go in. I lightly lapped one side on a piece of 320 grit paper adhered to a surface plate. Total material removal was probably somewhere between 1/2 to 1 thousandth. The rear infill once again went right in with steady pressure.
“When I was 5 years old, my mother always told me that happiness was the key to life. When I went to school, they asked me what I wanted to be when I grew up. I wrote down ‘happy’. They told me I didn’t understand the assignment, and I told them they didn’t understand life.” – John Lennon