Dee, let me note what I do w/ BIAB:
I have a 10-gallon kettle. I usually fill it with 7.25 gallons of water, adjusted as necessary w/ salts, gypsum, lactic acid, whatever.
I bring that up to strike temperature--which in my case, with my system, is about 159 degrees. Once reaching that temp, I turn off the burner and line the kettle with my BIAB (I use Michael Wilser's bag, it's great!)
I then stir in my crushed grain which is crushed finer than if I were using a mash tun and sparging, I make sure it's mixed in well (no splashing!), cover the kettle and then cover the entire thing with a sort of quilt-type thing to keep the heat in.
I stir at 15- and 30-minutes to ensure a good conversion. This helps--do it.
Then the only remaining thing is hoisting the bag to drain out the wort. Having something above to which you can hook a pulley is important for this.
I also squeeze the bag as it's suspended over the kettle to get out as much as I reasonably can. I use a pair of rubberized gloves to do this (well-washed before first use!) because, well, that stuff is still about 150 degrees.
I tend to end up with about 6.7 gallons of wort after starting with 7.25 gallons. That boils down just fine to about 5.5 or 5.6 gallons or so, a perfect volume for a 5-gallon batch.
Good luck, and enjoy the process! There's magic in the mash!