Hola Diego! I did this in Valencia last year. The registration itself was one morning. The hardest part was everything after: understanding which invoices needed which tax, and the quarterly declarations. My advice is to get a gestor, sorry, an accountant, from the first month. I tried to do it alone for one quarter and spent more time on it than on my actual work. Ojalá someone had told me that before.
Now that I'm thinking about moving from the nomad visa to regular residence, I'm also looking at registering properly as self-employed in Spain. I've read the steps and they look manageable on paper: register with the tax office, register with social security, choose the activity code, start invoicing properly.
But I know from experience that "manageable on paper" means very little. For those who registered as self-employed in a new country, which part was actually the hardest? The registration itself, the quarterly obligations, finding an accountant who understood foreign clients, or something else entirely?