To become self-employed you must register your activity as a private firm, which is done at the Bolagsverket (Swedish Companies Registration Office). You will also need to apply for a F-Skattesedet tax form, as you will need to pay your own monthly tax and social insurance fees.
To qualify as an Enskild Firma you must be independent and are required by the Swedish Tax Authority to have more than one customer. If you have a single client this may be considered by the tax authorities as an arrangement for a company trying not to pay tax contributions by not creating a job.
As a self-employed person you will bear all responsibilities for profit and debts incurred. You will be required to keep accounts and pay tax on the full year income. If your company has less than 10 employees and a net turnover inferior to 24 million kronor, you will not be required to have an accountant.
Partners can also be self-employed through a trading partnership (Handelsbolag), where all partners are liable for all agreements and debts. A limited trading partnership is a partnership where some of the partners only have a limited liability. At least one of the partners will need to be liable for accumulated profits and debts.