Look at your numerical data. What is the highest power of ten (units (ones), tens, hundreds, thousands, millions, etc.)? Now you need to make sure that all the numbers in the dataset are standardised to that highest order. That may mean putting in leading zeroes. So if, for example, you had numbers between 0 and 99 then the highest power of ten is the tens order. If you have a number less than 10, put a zero in front of it, so 7 would be 07.
The stem is simply the first digit of the data. So in this case you may have stems 0-9 representing the tens place. The remaining digits form the leaf. So in your stem and leaf table you put the stems at the beginning of each row. These start the horizontal lines of the table: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Now put in the leaves filling up cells along the rows. So, for example, 07 would be in the first row, stem 0. It would occupy the next cell on the right of the stem. If you had, for example, 23 and 29 in your dataset, you would go to row stem 2 and put 3 and 9 in adjacent cells.
Fill the table with your data just as it comes. The placement of the leaves forms a shape. There are more cells filled for some stem rows and they project further to the right than stems with fewer leaves. This is a frequency plot.
Now tidy up the table be arranging the leaf cells into numerical order along the lines starting with the smallest. This makes the plot more useful to draw information from.
If the data has a large range, the stems could be, for example, all the numbers between 0 and 49, 50-99, 100-149, etc. The same principle applies. You just put in the whole data for the stems along the stem rows. But usually you will find that the first digit idea is the most common one for stem and leaf plots.
Think of the stem and leaf idea as a filing cabinet. Just as you might have all folders for subjects beginning with A in a drawer marked A, folders for B-subjects in a drawer marked B and so on, all in alphabetical order in their own drawers, you can think of each drawer as a stem row. The filing cabinet can be thought of as the stem and leaf plot where the contents of each drawer are the leaves. Some drawers will contain more folders (leaves) than others. If your filing cabinet were made of glass you would be able to see which drawers contained the most folders and which the least. Stem and leaf plots use numerical ordering and this is similar to alphabetical ordering in a filing cabinet or library catalogue.