LESS and CSS and CUSTOM.CSS

Hi,
Firt of all i'm so sorry for my english but i don't seak it very well!!!

i'm Andre and i'm very new about T3.
Today i dowloaded for the first time t3 framework.
i installed joomla 3.3.6, t3framwork with t3_blank and t3_bs3_blank, i selected t3_bs3_blank like primary template, i enabled Development Mode and here is my problem!

i don't understand how is overriding less or css file and when need to use custom.css and where i find it?

i've see a lot of videos and readed a lot of documentation, but really i don't understand!

i have understand: when Development Mode option is enabled, t3 use less file, when it is disable use css file.
i understand that is better use less file... but i don't understand how can i do!


with firebug? with css? with less? when i need enable Development Mode and when i need disable it?

i'm so sorry for my english yet!

thank you very much in advance

Andre