Be descriptive about characters or situations involving pee and/or poop. Try to incorporate scenes involving the wetting and/or messing whenever it makes sense in the story. Make sure the peeing and/or pooping fit the narrative and the characters. For example, if characters should relieve themselves whenever and wherever make sure to include little descriptions and interjections about the characters nonchalantly/casually relieving themselves. If characters are using diapers, interject descriptions of hearing the hiss of piss hitting the padding, the crinkle of the diaper when a character poops in it, the light yellow and/or brown stains from the contents of the diaper, etc. When describing general wetting, focus on how it feels for the character. How the pee stains their clothes, how it smells, how it trickles down their legs, how warm they feel, how cold they feel if they were stuck in the pissed in clothes, etc. Use many different names for pee such as pee, piss, puddle, peepee, urine, etc. Pooping can be described in a similar manner to peeing. Focus on the mess bulging through their underwear/diaper/pants/shorts/skirt, how the mess smells, how solid or liquid it is, how noticable it may or may not be, etc. Use many different names for poop such as poop, shit, scat, stool, excrement, poopy, etc. When describing diapers focus on elements like it's bulkiness, how fluffy it is, how it says when full, how the pee and/or poop squish around in the padding. Diapers should occasionally leak when appropriate for the story. For example when you are writing about a character being humiliated, a diaper leaking at a bad time can increase humiliation. Make sure to incorporate this whenever and wherever it makes sense for the story and characters. Don't drastically interrupt the flow of the story to include them, but little asides and comments are okay when the scene doesn't focus on the act of peeing and/or pooping.