I assume you're using Word to do this? What version?
The process is something like this
Start a new blank document.
Go to the tools menu in word 2002, there is a letters and mailings option then a envelopes and labels option
If you select this, you need to select the size of labels you are using, there are hundreds of sizes, I'm sure that whatever you are using it will be there or the Avery equivelent will be there.
Once you have the right labels selected, Word creates a table which fills the page with the cells the right dimensions for your labels