After locating a variable star, many observers estimate its apparent magnitude. There are various methods, but most use an interpolative method, comparing a variable's brightness with that of nearby stars.
The American Association of Variable Star Observers (AAVSO) provides finder charts for individual variables that show the magnitudes of surrounding stars. Say the variable is not as bright as comparison star A, but is brighter than comparison star B. From your AAVSO chart, you know that star A is magnitude 8.5 and star B is 9.0, so you might estimate the variable's magnitude as 8.7.
The British Astronomical Society's method also uses comparison stars, but you do not need to know the magnitudes of the comparison stars until after the observing session.
Observing a star that is too bright for your telescope may result in an inaccurate magnitude estimate. A 4 inch (100 mm) telescope should not be used on stars brighter than 7th magnitude. The limit for 6 to 8 inch (150 to 200 mm) telescopes is magnitude 8.5. You can cut down the aperture of your telescope by making an aperture mask.
Some observers photograph variable stars. This provides a permanent record and allows them to estimate the magnitude away from the telescope.
For precise measurements of a variable's brightness, amateurs can use a photoelectric photometer, a device that attaches to the telescope's focus. CCD technology can also be used for photometry.
Groups such as the AAVSO or the Variable Star Section ot the British Astronomical Association welcome visual, photographic, and photometric observations from amateurs, and can provide further advice about variable-star observing.