Can someone refer me to some offline dictionaries to use with GoldenDict and how to install/integrate them with it? Preferably English to English dictionaries with large wordbase and phonetic notations for pronunciations, their usage in sentences etc.
There is a whole page dedicated to dictionaries on the official website, Getting dictionaries for GoldenDict. I'm not sure if you can get other suggestions here because, as clearly explained on that page, there is a risk of copyright infringements for the non-free dictionaries.
The best offline free English to English dictionary I know is WordNet. There is a version of it, specially formatted for GoldenDict, on the download page: search for WordNet, download the .zip file, unzip it somewhere and add that folder as a source for dictionary files.
Babylon offers a large selection of free
.bgl dictionaries that work flawlessly with GoldenDict.