Where did the premixed bags come from?
They are mine. Err... were mine. They were bought from a previous brewer, and I brewed a few recipes off the bags and here is what I did.
Weigh the bag, make sure you are using approximately the right weight of grain for that 19L batch. This will control your OG. Have a bag of DME/LME on hand to make last minute adjustments in case you undershoot the target gravity. If there is more grain in the bag, pull some and put it in a zip top bag, and save it for another batch.
Water. Most grains absorb about the same volume of water, so generally speaking your mash/sparge/pre-boil volumes are about the same for a 19l batch with the same starting weight of grain.
Sort the hops you got into 3 piles, bittering, aroma, and dual use. If you PM me your email, I can send you an excel sheet with the inventory and I've already got them sorted, and weight tallies.
Look at existing recipes for an idea of the type, timing, and amount of hops to use. You probably already know your hop preferences in various styles, but you can and IMO should experiment. Styles matter somewhat, but if you are not submitting for a contest, or making commercial beer that you want your customer to know whats in the can, then take some licence to have fun. Make something you enjoy drinking, regardless of style.
Software like beersmith, will be helpful to calculate your IBU's, You may want to de-rate the alpha-acid on the hops by 10-20% given their age, or not depending on your love of the hop flavours.
The water here in Alice Springs is hard as F^$#&^. I generally used 50% RO and 50% filtered tap. Using the water calculators that got the mineral content into the 'reasonable' range. If you want a deal on a RO system... let me know =).
Have 2 beers you are wanting to brew. The day of, or perhaps before, open a corner of the grain bag, and taste a few of the grains. If they taste good, and are not stale/mushy/off then you're likely good to go, if the grain doesn't taste 'good' don't brew with it, you're probably going to make crap beer.
I'm amazed how many brewers don't taste their ingredients. It really does help give you an idea what flavours each grain of the recipe contributes.
I've got a copy of beersmith, so if you want to sit down one night over here, and craft a few recipes let me know.