Upvote:1
See this question for details.
But in short, include information for all income you have. Copies of contacts, signed letters that this person employed you and paid you X money.
See this comment by @traveller
Start getting evidence for all income and put off your visa application until you have several monthsβ worth. There are many examples on TSE of refusals due to unexplained/undocumented deposits, 20% is a significant proportion.
Write a cover letter to include this information if there is not enough space on the form, but if at all possible make a note on the form where the information about that money is.
A tax form on which you declare that cash in hand money will help as well, if you do not yet pay tax over it, consider doing so, to avoid problems in the future.