Spend a feat on "Single-Class Paragon" and make sure that it's noted on your sheet in the area with the other feats.
As far as the new Special Abilities go, I'd use a label for each one in the Special Abilities area above the feats.
For example, if you were a Ranger: "Fast & Final Ambush", "Flanking Strike", etc.
As far as the enhancements to your normal class abilities (like the favored enemy enhancements for a Ranger, etc), in many cases putting them on the sheet isn't practical. It's best to print a copy of the Single-Class Paragon rules for your character class, and keep it with your character sheet. That way you can just refer to it when necessary.
You could also use the Notes section on the 2nd page of the character sheet to make a shorthand list of all the favored enemy mods you've accrued, and you can make relevent entries on the combat section of your sheet such as "Rapier, +3 attack, 1d6+2 dmg 1d6+5 F.E., crit range 18-20, 17-20 F.E." (in the case of ranger favored enemy bonuses) or some other way to make it easy to reference.