Eleanor Burns method is the best and I use it all the time.
I have both of the geese rulers, they make it much easier to square up.
She skips some of the instructions sometimes. One thing you do is make sure that both fabric squares have the same grain stretch matched.
After the final cross cut and you have 4 geese triangles fold the triangle in half and clip to the stiches at the fold then you iron the seams away from the goose so it lays flat nicely.
Then you trim with the ruler for the size you made.
There is a formula for figuring the size squares to use in the Star Spangled Favorites video. You will need to have QNNTV to watch and it costs 24 bucks a year.
Her website has changed so go to the upper right corner of the screen where the search button is and type in video that will take you to a screen where you look at the options on the left of the screen and pick television and video. That will take you to the video choices screen.