First of all, a good set of scale plans is helpful, but not mandatory. What is needed is a ruler, calculator, and accurate dimensions of the actual vehicle, such as length, width, height, etc. If you don't have reference material on the subject vehicle, you can usually use the vehicle dimensions included in the kit instructions. These dimensions are typically taken from published references, but I have seen occasions when the instuctions had incorrect information. So if possible, get a reliable reference book with accurate dimensions (and it will usually have scale plans as well).

The scale of the plans is irrelevant, as long as they are drawn well, and if you have acurrate vehicle dimensions. It doesn't matter what scale the plans are supposedly printed in, because chances are, they are incorrect. Trust me, I used to work in the printing indistry, and I know how difficult it is keeping correct scales while laying out page artwork. Always double check the plans' scale before relying on them.

Calculating the scale of a drawing is a very simple matter. Divide the length of the actual vehicle by the length of the vehicle's drawing, and the result is the drawing's scale (for the length). For example, an M-18 tank destroyer has a length of 6655mm (I like to use metric for ease of calculations); if the length of the drawing measures 92.5mm, then the drawing's scale is 1/72nd (6655mm / 92.5mm = 71.9). You should do this for length, width and height, but sometimes height is harder to measure than the other two. Sometimes you'll find that you get different scales for each measurement (i.e., 1/72nd for length, and 1/74th for width), so it's up to you how to deal with that. If they are way off, then either the drawing is completely incorrect, or your original dimensions are wrong. Either way, it's then best to find different dimensions or drawings.

Once you have the scale of a drawing, you can then use the drawing to scale out smaller parts of the kit, such as turret, wheels, hatches etc.

It's important to measure exactly the outermost parts of a vehicle when making these measurements. For example, for some German vehicles, the width measurements include the schurzen; usually, this will be noted in the vehicle statistics. I've seen some books that give width measurements with and without side skirts. Same goes for length; some measurements include the gun barrel, others do not.

I usually do not rely on drawings for scaling models, because there is so much room for error during the drawing and printing process. I only use drawings if I can confirm their scale from printed measurements.

Scaling a model kit is done in exactly the same manner. Divide the length of the true vehicle by the legnth of the kit (and width, etc.). Conversely, you can get an estimate of the kit's scale by comparing it to a true 1/72nd measurement. In other words, divide the true vehicle's size by 72. For the above example, we see that 6655mm divided by 72 equals 92.4mm. This means that a 1/72nd scale M-18 should be 92.4mm long. So if your kit measures 95mm for example, you know it's larger than 1/72nd; or it's 90mm, then it's smaller than 1/72nd.