Check the sub-floor moisture content; it should be no higher than 12% on a Protimeter prong test scale. Different meters will have different scales so you must always refer to the manual to ensure the correct reading. Now check humidity levels within the room using a hygrometer. They should be between 45-65% RH (Relative humidity). The sub-floor must also be perfectly level. An uneven sub-floor is another common cause of quality flooring installations failing. Use a spirit level and ruler to ensure that the floor does not have more than 3mm of dips or humps over a 1m length.
If making your own, they should be 3/16" to 1/4" thick, L-shaped, and a foot long or so. You will need several, six or more for the first two rows.
Make sure to check from the edges of the floor boards to the edge of the wall to make sure that the row is even with the walls.
Use a sharp miter saw to cross cut the boards shorter and a table saw to make them narrower. It may be easier and provide better results if you cut the boards upside down.
