Or just go to the drug aisle.
Get a bottle of "Magnesium Citrate Oral Solution" In whichever flavor you can keep down. It's inexpensive, and so it's a good idea to buy a second bottle in another flavor if you can, so you get a do-over if it doesn't stay down
Put that Magnesium Citrate in the fridge overnight.
When you get close to bed time, don't eat any solid food, alcohol, caffeine or dairy products for at least two hours before bed, but DO take any prescription meds you may be on.
When you wake up, don't eat any of the above. DO drink ALL THE WATER YOU CAN STAND!! Do take any prescription meds.
At least an hour later, find a large drinking cup, like a stadium cup or similar. Put cold water in it. Ice can help, but not strictly necessary. If you have never had the stuff before you don't know how your body will react, so you might want to put a pillow on the floor in front of the toilet, kneel on it and drink the stuff that way, in case the gag reflex is too strong.
Once you break the seal on the bottle it's time to get on with it. Drink a sip of the mag citrate and a sip of the cold water. Let the bitter aftertaste get past it's peak and repeat.
Keep that up till the bottle is empty. Then drink as much water as you can stand.
You could have as little as 45 minutes, or as much as 3 hours, but things will start happening in your stomach within an hour. Drink water any time you can, even if you have to just sip it.
There will come a point about 2-4 hours after you take it where you may not be able to leave the bathroom between expulsions.
when the expulsions start, they will look like pond scum and will feel like the worst enema expulsions you have ever had.
Things should start to wind down in about 6 hours, and be all done in about 8.
After that drink some Gatorade, Powerade, or Pedialyte to restore your electrolytes.
Inexpensive, and duplicates the expulsions of a high volume enema.