Before any kind of anal sex such as fucking, fisting or finger play most guys choose to perform a lavage to remove unwanted obstacles from the rectum. The rectum is a delicate body part so it is vital to conduct the cleaning gently.
Use body temperature water and do not use ding soap as it will only irritate the rectum. Do not push excessive amounts of water in at one go. Good tip is to lube the tip of your douche for comfortable insertion.