Rough bill/ invoice

In an institute there are many rough bills/invoice on paper from small businesses or grocery shops or tea and other materials purchase and small repair
what supporting is appropriate for the same
1 .rough invoice and Bill are appropriate in income tax
is yes than what are things that must be written in that
2 .simple voucher can be a legal document if yes than what were the things that are present in it
3.also as GST date for unregistered dealer goes to SEP 19 now what is the format for that dealer whether in entity we can accept there bills the rough bills simply on plain paper cutting on which item are written

kindly advice for audit and accounting of same

Thank you
waiting for reply