It sounds weird but actually using the bath isn't so bad, it takes me about half an hour to wash about five or six machine loads because the bath's fairly big compared to a washing machine.
Step 1: Hot water, Washing powder.
As for water you'll only need to the immersion/heating on about a third of the time to get enough to fill for washing.
I only filled the bath a bit in my photos because I didn't have to wash as many clothes as usual, I rough guessed the amount to be around 40L, so I poured in what felt right, it'll vary powder to powder but a rough estimate should be OK, if you go really overboard you'll need to rinse your clothes a few times before drying.
The temperature doesn't need to be anywhere near a normal bath, unless you like them lukewarm,