As an experienced recruiter, how do you deal or write-up performance problem of newly hired employee when it happens during the probationary period?

I would ask the employee if he / she is finding any difficluties in areas that have indicated his / her low performance. I would then provide feedback and address a solution for the same either with training sessions on his Key Accountabilities and  then follow up on his performance.

I personally talked to the involved employee and lay down first the expectations and objectives specific for his/her position. Make sure that he/she understand and answer all his/her inquiries. Afterwards, I will discuss the problem regarding his/her performance and provide mentoring if needed. The person needs to understand what is expected on his/her part so that person will be aware and will avoid same instances in the future. An initial warning will given unto him/her.

a prompt feedback by HR person and random evaluation by superviosr would be support in identifying and overcoming the gaps and difficulties. capacity building session would be the second step; if required.