To start calculating your cash outflow, look at the cash you need to pay out each week, month or quarter.  Fixed costs will include the costs of running your business premises: rent or mortgage and rates, energy bills, telephone, administration costs (including clerical salaries) etc.

Variable costs will depend on how much you make or sell: materials and parts, production and labour costs, packaging and distribution etc. 

Remember that there are further costs of sales to take into account, such as sales salaries (fixed) , commissions (variable) and marketing (optional) costs.

Plot the cash expenditure on a spreadsheet, keeping the fixed and variable costs separate but giving monthly totals.  This allows you to see the effect of increasing or decreasing the variable costs.  Take into account the normal credit period allowed by your suppliers which will determine when you have to pay for purchases and services.  Note that the Inland Revenue and HM Customs & Excise will penalise late payments, so include tax payments in the correct month due.