There are certain situations where you really cant find a temple or purohit to perform upakarma. here i have uploaded document from shri sudha magazine’s simple upakarma procedure at home. Please find the procedure for rig veda upakarma as well as yajur veda upakarma in tamil
However it is not recommended to perform upakarma every year at home. Whenever possible follow proper procedure and do it with the help of achars.
Upakarma means beginning. And it is considered as the auspicious day for beginning the vedic studies.
Rig veda Upakarma is observed on the day in/on sukla paksha Shravan month which has Shravana nakshatra
Yajur veda upakarma is observed on sravaana maasa hunnime(pournami/full moon day)
Document source: sri sudha tamil magazine