Ok, from the way you talk about green stuff, it seems like you haven't used it much yet (if that's not the case, please excuse me, i don't want to offend anyone).
Green stuff is as complicated as you want it to be, though i would start with the basics instead of jumping to convert an existing miniature into another one. Start small, try to do something easy to get the feeling of how to do it and how to use the green stuff.
This is a video that covers the basics with green stuff, but there are a lot more (from different youtubers, that cover different things ranging from making specific things such as robes, feathers, spikes and scars, to basically sculpt your own miniatures) on youtube:
This is a youtube playlist from Modern Synthesist that also covers different things, like the different sculpting tools and their uses:
https://www.youtube.com/playlist?list=PLyjaTOn1cvVL6sMEFQGSh7qU71zuO8ohJ
This site specializes in (obviously) green stuff and sculpting tools, and some other useful things that you may need or want while sculting or converting models:
http://www.greenstuffworld.com/es/13-materiales-para-modelar
The Army Painter also sell green stuff and sculpting tools:
https://shop.thearmypainter.com/products.php?ProductGroupId=5#The Original Green Stuff
I would advise against the liquid green stuff, i bought one bottle years ago, and the only use i found for it was for the bubbles in the first years of GW resin and for the spaces that appeared when 2 parts of a miniature don't match after glueing them together.
And i myself prefer to use the green stuff in bars, since the yellow part and the blue part don't touch.
I hope this helps.
P.S.: By the way, for sculpting tools, if you want to go cheap, just use toothpicks and a little water, the pointy edge of a nail, a pin, your wet fingertips... you'd be surprised what you can accomplish with that
P.S.: This is another youtube playlist for sculpting tips that is a little more advanced:
https://www.youtube.com/playlist?list=PLMtDX54Ksm_lZurGX_-T9mLl2weJXmZVB
