There is free recipe software available on the internet or you can purchase and download software for a free trial. It's inexpensive and worth having.
It depends on the malt you use and your efficiency but basically a pound of pilsner malt comes up to about 1.005 OG in 5 gallons (post boil/flame out). So if you wanted to brew a 1.050 batch just start with 10 lbs of malt (.050 / .005). How close you actually are to that gravity will depend on your mash efficiency and mash yield which is largely based on your individual malt crush among other things.
What everyone has said so far is dead on accurate! I’ll second the software comment and say a very useful exercise would be to go to brewersfriend online recipe calculator and play around.
1-listen to old jamil show or brew strong episodes on the brewing network to learn how to construct and think about recipes.
2-go play around with a recipe calculator while referencing BJCPStyle guidelines as they have an ingredients reference.
3-my one big tip is that most recipes are 75-95% base malt and the simpler the recipe the better... usually. As you learn/experience different grains it becomes easier.
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