Your post is a bit confusing. You say your big problem is that you need to make the posts "square", and you then go on to say you do not have a laser level. I assume you are planning
A laser level would be used to level the tops of the posts, not to make the tubes square. If by "square" you mean plumb, you plumb the tubes with an ordinary level, check three sides, if the tube (I assume you are using sonotubes) checks plumb on three sides, it is OK.
If by "square" you mean that the tubes are correctly layed out in the horizontal plane, you use the technique previously described by concretemasonry. I normally set the outside lines of the tubes along the centerline of the tubes using the 3 4 5 right triangle principle, then measure off the corner tube to set the interior tubes using an ordinary tape measure. You can use string line between the marks to align the interior tubes. You can also use the string line to cut the sonotubes off at the correct elevation. Use a string line level to do this.
As for sizing the tubes, you do not indicate that you have any questions about that, so I assume you have selected the required tube size and spacing based on code or building inspector recommendations. Similarly the required depth of the footings should be specified in your local code, and possibly the required method of attaching the post to the footing. Design of the deck itself is also likely in the code.