Tell the truth - you love dressing up like a woman. You love panties and dresses and skirts. So, why?
It wouldn't be much of a truth if you kept it to yourself; post a short explanation somewhere with a little permanence - something that will be around for at least a day. You can do it anonymously, just write it somewhere. "I love dressing like a girl because..." You can even post it here if you can't find chan board or forum or website that suits you.
Go to a convenience store or department store and purchase a box of tampons for yourself. Oh, but it's not that simple, sissy. You must wear some makeup, you cannot use a bag, and if anyone asks you must tell them that they are for you.
When you get home use one of your tampons (hopefully you bought size-appropriate tampons). You can leave it in for a few hours or take it out right away. Of course, if someone comes over and asks about them you'll have to tell them that they are for you.
Boring (and potentially gross), but informative safety information: Some sissies (and girls) have concerns about toxic shock syndrome from tampons. TSS stems from the build of toxins released by bacteria where it can enter the bloodstream (not necessarily the tampon material, but more absorbent tampons means more moisture). The bacteria can grow in warm, moist areas (which is why it's better to store your tampons in a hall medicine cabinet instead of a bathroom); TSS in relation to tampons happens from the collection of menstrual fluid and the heat of the human body. You can decrease your already minimal risk by washing and drying your hands before inserting, using less absorbent tampons (although there really shouldn't be much fluid to absorb from your rectum, sissy - I told you it could be gross), and changing out your tampons frequently.
Hypochondriac sissies may substitute a buttplug for their tampon.