So, you know French and Spanish and want to learn Catalan? English and German and want to learn Dutch? Polish and Russian, and Ukranian? It should be dead easy, right? All you need to do is start reading and you'll get it.
Possibly, but it's still worth picking up a book and doing a bit of study.
Eliminate the negatives:
If you just start reading (and/or listening), you will develop a reasonable passive understanding, but there's a couple of ways this limits you. Passive understanding does not require you to process all the language in front of you (you can gain complete comprehension without complete perception) so you are never forced to develop an accurate internal model of the language.
This can be very limiting, because in the future, if you decide to learn to speak the language then you're faced with a massive "frustration barrier" -- a lot of people find that being able to understand lots but not answer is a very unpleasant situation. It seems more discouraging to me at times than simply having low skills all round.
Accentuate the positives:
Besides, the most important thing stage of learning a related language is the very basics. How so? Well this is about the nature of regular and irregular language forms (as I've been talking a bit about recently).
Irregular forms are almost always the most common forms. So in English "child, children" vs "adolescent, adolescents"; "give, gave" vs "donate, donated".
Interestingly enough, it's those most common words that are least stable. The English word "will" (I will go, etc) developed from a word meaning "to want" (compare Modern German will and the Modern English noun will -- eg strength of will, willpower), whereas "want" originally meant "to lack" (they found him wanting, ie inadequate).
The same effects can be seen in other language families -- while the Italian and French words for "to have" come from the Latin "avere", the Spanish word for to have (I have a car etc) is derived for the Latin word to hold -- "tenere". But when we get to a rarer word like "cultivate", we have an almost identical word: cultiver(FR), coltivare (IT), cultivar(ES).
So when a Spanish person says they can "understand" Italian or Catalan without ever having studied it, they genuinely believe that they can, because they can understand what they think are the "difficult" words, but are in reality the easy words.
The mistake most people in this situation make is to skip the beginners' material and jump straight to the advanced. But it's the beginners' material that teaches most of the things you really need to learn.
A little bit of time dedicated to the basics (conjugations, pronouns, declensions) at the start will accelarate you through to 90% understanding very quickly.