First of all, let him know what you're going to do. Just say, "Dad, I'm going to put a bedpan underneath you," and cue him verbally step by step. Assuming he's lying on his back, ask him to try to bend both knees up, if he can, and have him lift his hips off the bed to enable you to move an absorbent underpad (a disposable pad that protects the mattress) and the bedpan into place. If he’s unable to do this, help him bend both knees up and have him turn toward his strong side. If his left side is weaker, for example, have him turn toward his right. Encourage him to let his knees fall to his right side, which will help with the momentum of turning. If he has difficulty, you can just bend his knees, hold his knee and shoulder, and roll him over onto his side. Wedge an underpad beneath him, have him roll back and try to position the pad so that it lies flat. Have him roll again and place the bedpan underneath his bottom, so that when he's ready to roll back, he's right where he needs to be. Let your father rest in between movements, and check with him to make sure he is not overexerting himself. Take your time.
When he's finished relieving himself, have him roll over again onto his side, so you can slip the bedpan out and clean his bottom. Most people find it’s easiest to use a moist wipe with a gentle cleaning solution. (Examples of brands are Tranquility cleansing wipes and Tena skin care washcloths). Make sure to wipe him from front to back to ensure that he stays clean and avoids infection, and keep the area dry to avoid skin breakdown.