A budget starts with what really comes in: your net salary, not your gross. On €30,000 gross in Cataluña, that is €1,924 a month in 12 payments.
The 50/30/20 rule
A simple starting point:
50% for needs: housing, utilities, food, transport, insurance.
30% for wants: leisure, eating out, travel, subscriptions.
20% for savings and paying off debt.
On €1,924 a month, that is roughly €960 for needs, €575 for wants and €385 to save.
Adapt it to your life
In cities with high rents, needs eat up more than 50%. That is fine: what matters is that savings are not what "is left over" but the first thing you set aside. See the savings rate.
If you are paid in 14 payments
Budget with your normal monthly payment and treat the extras separately, for yearly costs or savings. See how to plan extra payments.
If you are self-employed
Your income varies month to month, and you also have to set aside the cuota and IRPF. Budget with your average monthly net, after tax, and keep a buffer of several months of costs.
Start from the net
The calculator has a savings section: enter your monthly cost of living and see how much you keep a year and what share of your net you spend. Try it.