Find out what tabs look like and how to read them at ultimate-guitar.com by typing "How To Read Tablature" into the search bar on their home page and then selecting "Lessons" from their drop down bar located right next to the search bar.
Study your sheet music, then write down the names of the chords or notes in the order that they appear in the song on the first sheet of paper. Organize the piece by its structure. For instance, keep the verses and choruses separate, and then write down the order in which the song is constructed (for example: verse, chorus, verse, chorus, bridge, chorus).
Draw your guitar tab skeleton on the second sheet of paper. Make a section for the verse, chorus, bridge, etc. Draw only the names of the strings to begin with. Don't yet write down any numbers.
Find the corresponding numbers on the fretboard of the guitar to each of the chords or notes you have written on your first sheet of paper. For instance, if the first thing you wrote down on your first piece of paper was a standard G chord, then draw the tablature symbol for the G chord right above it. This will help keep you organized until you're ready to make the tab.
Put the numbers in their correct order on the second sheet of paper and in proper tablature form. Review your sheet music to make sure the tabs read correctly, and that you haven't left anything out.