I want to lend money to a friend. How to competently arrange it legally so that ...?

The most simple and effective way is to issue a debt receipt.

Receipt sample:

I, Ivanov Ivan Ivanovich, born on June 1, 1990, living at the address: _ _ _ _ _ _ _ _ (passport AA No. 123456, issued ___ _year), give this receipt to Petrov Petr Petrovych, born on April 1, 1994, living at the address: _ _ _ _ _ _ _ _ (VV passport No. 123456, issued __ __ year) about the fact that I received a loan from him in the amount of 40,000 hryvnias (forty thousand hryvnias) in cash. I undertake to return the loan by May 31, 2018, by transferring the above funds to bank account No. 1234567809908.

The receipt was written by me on April 15, 2014.

Ivanov I.I. ________

(signature)