Can you apply for Non-Lucrative Visa by yourself or best to do it with an agency?

Hi! I'm planning on taking a sabbatical next year and decided to do it overseas. Spain was the best option for me. I'm looking at the process on how to apply for it and I usually apply for visas on my own but not sure if this is a particularly harder visa to get.. If ever, is it still doable by yourself or do you suggest I go with an agency?

For those who applied for NLV, how was your experience? Was it straightforward or were there a lot of complications due to bureaucracy? Any advice or insight would be greatly appreciated!

Muchas gracias!