Whole bunch of ways to interpret those instructions. Doubt any of them will cause any big pluses or negatives, so do whatever you think is going to be your process. Mine is to add all the water, heat that to the mash temp, add the grains. Pull the bag when time's up, give it a decent squeeze and start heating to a boil.
Plenty of people change that by adding around 70% of the water, heating that to the mash temp, adding the grains. Pulling the bag when the time's up, drizzling the remaining water over the bag and start heating to a boil.
For the drizzle, some do it in a separate bucket and add that back into the main kettle. Others drizzle it over the bag while it's in the main kettle. Also some will use warm water, others will use room temperature water. You'll get a little bit more sugar with the warm water.
There's some slight increases or decreases in the amount of sugar you extract with these variations. Whether it's worth the effort is completely up to you. I doubt the differences in efficiency add up to more than extra cents, not dollars, in grain costs. The chances that any of these variations change the quality of your beer have got to be close to zero (i.e. it won't be the process, just your ability to do it consistently).
Though there's a few styles of beer or types of grains where targeting specific mashing processes can be important. But not something to worry about when you're starting out.
