Fatou, I had exactly this worry when I applied in Portugal, because my income looks like a heart monitor. I'll describe what worked for me, with the usual caveat that each country reads the rules its own way.
What I submitted, in the end:
- bank statements for the full period they asked for, not just the good months, because a gap would have looked worse than a low month
- a simple one-page summary that I made myself: month, client, amount invoiced, amount received. It made the average visible without the officer doing maths
- signed contracts with my two biggest clients, showing ongoing work into the following months
- a letter from my accountant confirming my annual income, which carried more weight than I expected
The contracts mattered because they showed the income would continue, not only that it existed. I wasn't refused, but I was asked one follow-up question about a low month, and the summary page meant I could answer it in two lines.
My honest advice: check the official requirement for how the income is assessed (average or every month) before you choose the country. That single detail decides whether your uneven months are a problem or not.