Sammā Sambuddha Saraṇaṃ,
This is a good presentation explaining Bhikkhu Upasampadā (upasampadā vidhi) written by Somdet Phra Mahā Samaṇa Chao Krom Phrayā Vajirañāṇavarorasa.
For reading pleasure of many 
https://drive.google.com/file/d/134h2iMSuWO0UJHdl0VbMBoWmPwTnoyRe/view?usp=drivesdk