Here is a short Pachchhakhan (formal religious vow) for Athai (a type of Jain fast where one does not eat for eight consecutive days):

Sanskrit/Prakrit Pachchhakhan for Athai:

Ichchhāmi paḍikkamāṇuṃ, samāiyāe paḍikkamāṇuṃ, savva pāvappaṇāsaṇāe paḍikkamāṇuṃ, athaiṇā athāiṃ uvāsagāraṃ paḍikkamāṇuṃ, tāe miṃ paḍikkami.

English Translation:

"I desire to take the Pachchhakhan, I take the Pachchhakhan in the presence of the Arihants, for the renunciation of all sins, I take the vow to observe Athai (eight-day fast) with full restraint, so I resolve."

You may recite this or a similar version before commencing your Athai, ideally in the presence of a Jain monk/nun, or in front of the image of a Tirthankara. The essence is to state your intent to perform Athai with sincerity and religious discipline.