Formally, the freelancer must be registered as individual entrepreneurs (self-employed). To obtain a Schengen visa in addition to standard set
it will be necessary to provide a copy of the certificate of registration and the original of the certificate on statement on the account in tax bodies.
Place of work entrepreneur is his own IP.
Post - an individual entrepreneur (individual entrepreneur).
Work experience - starting from the date of registration of the physical person as IP.
The amount of payment on the basis of the Declaration on the taxes SP (it is usually also asked to provide), you can estimate your average income.
In General, when the visa usually want to make sure that the income is sufficient to visit the country. Most likely will also need to provide Bank account statements (accounts) of IP or just your personal accounts / cards as individuals.
It is always better to know in advance what specific requirements for individual entrepreneurs regarding the documents makes the Consulate of the country you intend to visit.