After a wheel has been properly mounted on the machine, the next step is to do a centering check. The wheel is spun and location noted. Then it is loosened and rotated 180 degrees on the shaft and tightened again before being spun. This will enable to machine to determine if it has been centered properly or not. Most shops skip this step and it is not the proper way.
Then, it is figured out where to apply the weights, depending on the type. Some wheels use clip-on weights, some use stick-on weights, and some can use a combination. Stick-on weights are best because they do not damage the wheels in the long run, while clip-on weights can trap salt and debris and initiate corrosion, especially if they are hammered on or pulled off without proper care with the correct tools. Clip-on weights must be of the proper cross-section, for example, most Japanese steel wheel use the FN profile. A good shop will have the proper profile gauge and the necessary weights in several sizes to do a good job.
Some machines can even figure out how to apply all the weights from the inside so that no weights are visible from the outside, and also how to hide the weights behind spokes. Ask the shop if they can offer these options.
For stick-on weights, make sure that the surface is properly cleaned and dried before they are applied, otherwise they will not stick and may fall off, causing vibrations. Also make sure that all the old weights are taken off before the wheel is spun on the machine, otherwise you will end up with a ugly mass of weights on the wheel.