I use the Homebrand Lager tin from Woollies. I keep it in the bottom of the fridge door. It doesn't go off, as it's basically full of sugars and is similar to golden syrup etc in that regard.
135gr of goop in 1 litre of water gets me to 1.040 SG. For bigger or smaller starters, just calculate the proportion you need to use. For different SG, again just calculate it proportionately.
To make my starters, I measure the volume of water I want into a pot. The tin of goop is placed on scales and, using a spoon to get the goop out, I measure how much the weight of the tin goes down as I take the goop out.
I boil it for about 10 minutes, and tip it into my Erlenmeyer flask, and chill that in cold water baths in the sink. Very carefully add the stirbar (you don't want to just drop it and crack your flask). Then onto the stirplate.
I'm not aware of any particular thread on how to do this, but it's quite simple, and nowhere near as complicated as trying to understand the logic of some court's legal decision, as I had to do in a past life before retirement.