All I do is an Excel Spreadsheet with our income and outgoings.
I do one about every 2-3 months, updating it when we change our provider of certain products like utilities. I know exactly what we spend on some things each month and roughly what we spend on others, and they are things that are non-negotiable (utilities, internet/phone package, petrol, food), and then I allow a budget for general spending (clothes, going out etc) and that amount gets transferred from our current account to our savings account once a month.
I know my husband's exact take-home pay/petrol allowance, but he does occasional overtime and is given odd bonuses which we just guess as they are not regular. My own income varies a little and obviously with my maternity leave, I won't know exactly what I will be getting. I put those onto another side of the spreadsheet and always ensure we have about a £100 between what we have coming in and what is going out. To do this, I often have to completely cut things out.