My first brews many moons ago were made using a boiler/mash tun like those using a grain bag :thumb:
This is how I would do it.
Get another 25L brewbin and fit a tap to it. Secure your grain bag inside it, this will be your mash tun.
Heat your de chlorinated tap water to about 77C. Work out how much mash water you require at a ratio of 2.5l/kg i.e 4kg of grain 10 litres of water and add this to your mash tun. Place this on a table or similar.
Wait until the temperature of the water in your mash tun has dropped to 73C and slowly stir in your pre weighed grain to create your mash. Fit the lid and wrap it in a sleeping bag, duvet, tank insulation, blankets coats or anything to give insulation. Leave for 90 minutes.
Meanwhile heat your sparge water in you boiler. I would recommend a batch sparge for your first brew, use this
CALCULATOR to work out what you need.
Sparge into another fermenter to collect the sweet wort, once your boiler is empty fit your hop strainer add your hops and sweet wort and boil for the recommended time.
Strain cool and ferment :thumb: