Well, there are several ways of forcing diaper use for pee and poo.
First for pee. The obvious is to drink lots of fluids, especially diuretic ones, such as coffee.
Alternately, it is possible to fill the person's bladder with sterile saline then put on the diaper. Depending on how much is put in, the urge to pee can be mild to extremely urgent. The filling is done with a Foley catheter, inserted using sterile procedure, the balloon inflated, the saline given, the catheter clamped, and the balloon deflated, and catheter withdrawn, leaving the person full of pee. It is absolutely essential that sterility be maintained throughout, otherwise a UTI may happen and those can be really problematic.
If you are not completely certain about you ability to do this safely, stick with the coffee and other drinks. You'll certainly pee, but it will be later than sooner.
There are several options for poo, depending on the time you have available for play and the kind of poo you want to pass.
If you have plenty of time, like a day, an oral stimulant laxative is a good choice. Depending on what is taken and the dosage, the effect can be anything from a gentle hint that there is poo wanting out to gut wrenching cramps and a very, very strong urge that pushes the poo out no matter how you try to resist. My preferred laxative is Senna. One Extra Strength ExLax usually gives the mild urge, four is a moving experience, six a colonoscopy dose. Dulcolax can also be used in the same way.
The stimulant laxatives tend not to soften poo, so if you want your diaper to be full of mush, take a softener before and along with the laxative dose. For example, if you have a day to play, take a full dose of Miralax at breakfast, lunch, and dinner then at bedtime take the full Senna dose and another dose of Miralax. You should be rewarded with a very full diaper. Protective panties are highly recommended. The softness of your poo can be adjusted by the Miralax dosage.
If your time is more limited, Castor Oil will usually make you use the diaper within 6 hours or less and several of the saline laxatives (Epsom Salt, Milk of Magnesia, Glauber's Salt) can have you pooing in under an hour. These usually produce soupy poo which is a real mess.
There are also options given rectally- suppositories and enemas.
With suppositories there are basically there options: glycerin, soap, or Dulcolax. The last are the most powerfully laxative (especially the Magic Bullets). However, for them to really produce a good poo, they must be given properly... pushed well up into the recipient's rectum and placed between the bowel wall and any poo and moved around a bit after insertion, then retained until the urge grows to intense. This is hard to do solo, but a suppository applicator can help. Suppositories generally do not soften or lubricate poo... just make the recipient push the poo out.
Enemas can be either large volume or small volume. I'll call anything under 8 Oz (250ml) a small volume.
IMO, large volume enemas are petty much unsuitable for diaper play. For one thing, after the full dose is in the pooing can start very soon, even before it's possible to get the diaper on and taped and protective panties on. Also, the volume of fluid that comes out after a bag enema and the speed with which it comes out almost guarantees diaper failure.
Small volume enemas are a much better choice, IMO. If you're a newbie, try taking a Fleet then diaper up. This experience will give you a good idea of realistic expectations of such play.
My favorite way to play this game is a variation on the small volume enema. For this, if you want softer poo, several doses of Miralax the day before play is needed. For this game a Foley catheter, about Fr 18 to Fr 24 (preferably with a 30cc balloon, although the standard 10cc will work) is lubed and inserted rectally about half it's length. Be sure to place it between the poo and the bowel wall, not push it into a lump of poo. Then inflate the balloon with water. The person may feel the catheter somewhat, but it should not be uncomfortable. The diaper and protective panties are then applied with the catheter hose coming out above the waistband or though a leg hole. The person is now ready to be made to poo in the diaper on command. The Foley balloon helps to keep the catheter in place and will be easily expelled when the pooing begins.
When poo time comes, a small volume enema is given. These are usually 2 Oz to 6 Oz and are given with a piston syringe of about 150ml. Effective solutions include 50%/50% glycrein water; 20mg Dulcolax in 100ml water; a teabag of senna tea steeped in 6 Oz water; a soap solution; etc. Once the solution is given, the catheter is clamped and results awaited. Usually things will start to get interesting within a half hour.