You could always start with something simple like the Bell's Two Hearted Ale inspired clone recipe from Bell's General Store:
Batch Size: 5.00 gal
Estimated OG: 1.065 SG
Estimated Color: 6.2 SRM
Estimated IBU: 62 IBUs
Brewhouse Efficiency: 66.00 %
Boil Time: 60 Minutes
Mash Temp: 150F
10 lbs 2-Row Pale Malt
3 lbs Maris Otter
8.0 oz Caramel 40
1.25 oz Centennial @45.0 min
1.25 oz Centennial @30.0 min
1.0 pkg Safale S-05 (I prefer Bell's house yeast, but this works fine)
3.50 oz Centennial - Dry Hop (3-5 days before bottling/kegging)
How much water you use depends entirely on batch size and your particular equipment. Start by figuring out your boil off rate if you already haven't and use
Brew in a Bag (BIAB) Calculator ~ to get you started. Take good notes during brew day, noting your strike water volume, pre-boil volume and post boil volume (both hot and chilled) and volume into the fermenter. After a few batches, you'll be able to dial things in better.
There is a lot of free software out there (Brewer's Friend, Brewfather, etc) that you can use to make it easier to dial in your efficiency as well. Bell's recipe above seems to assume 66% efficiency, which might not be a bad place to start for your first all grain brew. It may actually end up higher than that, but it also could be slightly lower. Efficiency depends on several factors as mentioned in this thread already, but here are a few tips:
- Stir the grain a few times during the mash - I stir every 10 minutes. The down side to this is that you will lose heat more rapidly, but if you have the capability to apply heat while you stir, you can easily get it back up to where you need it.
- Double mill the grain - I personally don't, but this and/or milling finer seems to be popular among BIABers. Some people like their crush to be almost flour. I prefer more of a standard crush with some husks still mostly intact.
- Mash Out - After the mash is done, heat the mash to around 168-170. There is a lot of debate on this step, but in my experience, at the very least, it lowers the viscosity of the mash to help it drain better. Some consider it a waste of time, but my view is that you will be heating the wort to boiling temp eventually anyway - this just gives you a head start before pulling the bag. I also have gained a slight increase in efficiency by doing this, but that could also be a result of the extra stirring while I heat the mash.
Once you get more comfortable with the process and get your numbers dialed in, start looking into water chemistry. At first it can be a bit intimidating, but it's really not. Mash pH has an impact on conversion and efficiency, as well as fermentation.
A lot of BIABers also squeeze the bag after they pull it and gain some efficiency there. I personally don't squeeze ever and average around 78-80% mash efficiency.
Some BIABers also hold back some of their water to use for sparging, which will increase efficiency. It kind of defeats the purpose of the simplicity of BIAB, which is a single vessel, full volume mash and boil. I do sparge with about half of my brews, but it's mainly due to volume limitations of my brew kettle.
I'm sure I have left out something, but I'm on my third mug of coffee and have to pee.