Learn how to do a proper static test. It requires one line tied directly to the tip top in order to load the blank, and another line attached to the reel with a weight to keep some tension on the line. The technique is not difficult to learn, and what I use to place all guides on a casting rod and the running guides on a spinning rod.
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If you start at the tip and work backwards, the guides will end up where they need to be. Michael gives some good advice on guide sizing. Personally, I typically use a butt guide that is a double foot 6mm guide, and follow it with 5mm, or a single 5mm, and 4mm running guides on the freshwater rods I use. I prefer a spiral wrap on most casting builds, but use both spiral and traditional guides on top builds.
In regard to guide sizing, there are a few considerations since you didn't mention what you are building for. When choosing the smallest guides on your rod, you need to be sure that they will pass any knots and connections that you require to pass through the guides (generally 5mm or larger depending on the knots is a safe bet). If you regularly fish in temps below freezing, larger guides can be useful in preventing the guide from icing up (6mm or 8mm). For heavy lines and offshore fish in saltwater, you may wish to go larger. When it comes to choosing the butt guide, it needs to be tall enough to satisfy the static test on rods where a fore grip is not used by the angler. If you regularly manipulate the line by hand, i.e. flipping, the height should be tall enough to allow you to easily grab the line (10mm, RV6, or 12mm). If the angler uses a fore grip the butt guide should be tall enough and close enough to keep the line from touching the angler's hand when the rod is deeply loaded.
