If you put the grain in first, then heat up the water you'll end up ramping through all the various rest stages, when really you just want to jump straight to a beta amylase rest.
A fairly safe thing to do is heat your water to about your mash temperature, put your bag in (this will cool the water), then heat up to about 2-3F more than your mash temperature.
Then dump your grain, agitate with a potato masher and apply heat until you hit your mash temp again. You should be pretty close. After a few minutes agitation the mash should be a uniform temperature and pretty close. I would normally heat slightly warmer than I need, rather than cooler
And then you insulate and walk away.
If you want you can apply more heat at 15 or 30 minute intervals... up to you.
If you want you can then heatup to mashout temp (172F), and pull the bag.
If you are going to be applying heat while you have the bag+grain in there, then it is a good idea to use a cake rack or some other solution to raise the bag off the bottom of your pot, just a little bit.
If you actually pull the bag out to raise the temperature, the temperature will fall very quickly when you lower the now cooled bag back in!
The website for that "biab forum" is http://biabrewer.info