... or you can mash a small amount of grain to make starter wort in a thermos flask
170g of pale malt should get you something roughly equivalent to 100g of DME ... add it to around 350ml of water at around 70C strike temp in a pre-warmed 1ltr thermos, seal it up and give it a good shake, then leave it to mash for an hour or more ... empty it out through a sieve into a pan ... then put the grains in a bowl and rinse any grains left in the flask out and into the bowl with around 900ml of water at around 80C, leave that to dissolve any remaining sugars for a bit (that's a "batch sparge", and you could do it twice with half the water each time if you could be bothered) ... then empty that through a sieve into the pan with the first runnings, and boil the lot up until it reduces down to around 1ltr of wort (but at least for 15 mins) ...
that should get you a ltr of starter wort at around 1.040
... for the smaller amounts of lower gravity wort that are used in the instructions at the start of this thread you can take amounts of this and dilute it down to the gravity required (e.g. for 100ml of 1.020 wort, you can add 50ml of water to 50ml of 1.040 wort) ... you can portion any excess of this wort up and freeze it until you need it, but after defrosting boil it up to sterilise it (dilute it slightly to compensate for evaporation) before you add any yeast into it
Cheers, PhilB