BIAB is relatively straight forward. Firstly its "Brew" in a Bag not "Boil" in a bag. I made this error on my first foray into BIAB. Basically you put all of your grain makings into the bag which has been deployed into your boiler at around 72*C - be careful to stir it in as it might clump and form dough balls, this is because of the sugar contained in the malted grain. Once you have added all the grain, t this point I switch off the power to the boiler, this allows the temp of the liquor and the grain to drop to about 66*C which is generally the ideal temp for mashing in BIAB. Leave for 90 mins - the temp will drop a further few degrees but if you have insulated your boiler the drop will not be too much - my experience indicates this is around 62*C after 90 mns. I have left it for longer when it dropped to 58*C after 4 hours - I don't find this is a problem.
I then remove the bag, as has been said its heavy and this is not easy especially if you are brewing on your own to either get a pal along to help or organize some mechanical assistance like a pulley arrangement above where you are brewing to enable the lifting out of the bag. I leave it suspended above the Wart to drip out, once the drips reduce move the bag into a clean bucket with some sort of elevated platform in it to allow the bag to continue to drip.
You now need to bring the wart back to a boil as quickly as possible, you will also find that in removing the bag you have also removed some of the liquid out of the boiler, y9u can replace this progressively during the boil by adding boiled water and the recovered wart from the bucket where the bag is sitting.
I generally have another small kettle with boiling water standing by to enable me to do a quick sparge of the grain filled bag by trickling hot water (around 72*C) over the bag to rinse out any remaining sugars.
The rest of the process is pretty much the same whichever approach you take (BIAB or mash and sparge). The wart needs to boil for 90 mins then cool quickly down to around 25*C so you can pitch your yeast starter, this quick cooling helps to some extend by nt allowing any nasty yeasts into the brew. Once at around 25*C you can pitch your yeast starter and cover so the brew can get underway. If its a good going starter that you pitched it should show signs of fermentation fairly quickly.
It wont show much activity it you just sprinkle the yeast on the wart for possibly up to 48 hours, the yeast needs to grow to start converting the sugars into CO2. I don't recommend doing this I like to see a good going starter with a good covering of foam before I pitch it.
Good luck with your brewing and hope it works out for you. You know where to come if you have any questions.
