registerday

Brew in a bag is usually done using a single step infusion mash, the same profile most all grain brewers use.  This involves preheating the water in the mash tun to a predetermined temperature before adding the grains.  In a major departure from traditional methods, the entire pre-boil volume of water is used for the mash.
In BeerSmith, you can do this by choosing a single infusion, no mash out mash profile and then setting the first mash step volume (choose details next to the mash profile, then double click on the first step) equal to your boil volume.
You can also use the infusion tool to calculate initial strike additions, setting the strike volume equal to the initial boil volume for your batch.  For a partial mash BIAB, less water is typically used – but again it is equal to your starting boil volume.
Once the strike water is heated to the appropriate starting temperature, the bag is added to line the edge of the boil pot, and the grains are added.  Done appropriately, you should come very close to your target temperature for mash conversion – usually between 148 and 156 F.
Once you reach your target mash temperature, it is best to cover your pot and maintain the temperature as steady as possible for the next 30-60 minutes while the complex sugars in the grain are converted to simple ones.  You can also wrap the pot in towels to help maintain temperature.
After the mash is complete you have the option of heating the mash slightly to a mash out temperature (around 168F).  If you are planning to heat the pot while the bag is still in it, you do need some kind of screen or false bottom at the bottom to prevent the bag from getting burned or melted by direct heat from the burner.  For BIAB, the mash out aids overall extraction efficiency when you remove the bag.
Finally, slowly lift the grain bag out of the pot and let it drain.  Once the bag has drained you can empty it, spray it down and clean it off for reuse on your next batch of beer.
From this point forward, the wort left in your boil pot can be boiled, cooled and fermented just as you would any batch of beer.  If brewing all grain, simply boil the wort with hop additions, cool it and transfer to your fermenter.  For partial mash, you can add your extract, hops and continue to brew.