A cashflow forecast is how much you expect to come in, how much you expect to go out, and when you expect this to happen. You might do it by week or by month. It's useful because you might need to pay for things upfront before you get paid by your customers.
Month, Description, Cash, Running balance
January, Rent, -£50, -£50
February, Rent, -£50, -£100
February, Income, +£20, -£80
March, Rent, -£50, -£130
March, Electric, -£70, -£200
March, Income, +£120, -£80
April, Rent, -£50, -£120
April, Income, +£250, +£130
It will be more complicated than that. But as long as the values are realistic and you don't forget anything, it demonstrates viability. And in the very simple case above, tells you that you'd need at least a couple of hundred quid in the bank at the start.