It depends on the jurisdiction and whether you are a citizen or resident there.
In the EU, non-EU/EEA applicants are required to have a legal representative for all proceedings before the EUIPO, including filing. EU-based applicants can file independently. Similarly, in the US, only foreign applicants are required to use a licensed US attorney. On the other hand, in the UK, there is no requirement for a licensed attorney to file, so any applicant can lodge the application directly.
Where self-lodging is allowed, it is generally straightforward if the application has already been prepared correctly by an attorney.