In reading a buret, ruler, meter or anything with divisions, interpolate between the smallest division. For example if a metric ruler has 1 mm graduations, measure lengths to .01 cm.
By recording a number as, for example, 51.23 cm, you imply that the number may actually vary from about 51.22 to about 51.24 cm. This implies that there is the possibility of about a .02 % error in this measurement.
CALCULATIONS and SIG FIGS