I like to look at a budget with a sense of curiosity and run different scenarios. Very little is truly a fixed cost, there are just choices. Make a spreadsheet, plug in the numbers you know (rent, insurance, 401k, phone bill, gym membership), and then list out the categories whose numbers you don't know or that vary a lot so you don't forget they exist. List all the things, this is not an exercise in deprivation. Haircuts, charity, emergency fund, oil changes, vacation, running shoes, vet visit, additional investing, gifts, movies. Put the income in at the top and subtract. The number at the end has to cover all the things whose price was left blank. Then start messing with it. Add prices if you know them but don't forget that they are not actually static. If you max your 401ks, what happens? You want to increase your investing and go on vacation, but then your number at the end is negative. What if you cut out the gym? Your charity spending is less than your haircut spending. You don't like that, but you don't want to pull back on savings or fun stuff. Well, what could happen if you moved to a cheaper place? Put it all on the table with curiosity and without judgement and you will quickly learn some things and find out if there are some areas that need deeper conversation. Goals and values will naturally come up.
Then make any agreed-upon changes (downgrade internet, increase auto-savings, whatever) and track spending for a month or two. We use Mint to track from our one joint checking, 2 credit cards, and Ally saving account (which allows sub-accounts so you can keep track of vacation fund vs sinking fund for car insurance). Then make adjustments as necessary. You've probably forgotten some things, maybe cutting back the grocery bill was harder than expected. Be realistic, but stay focused on your goals. Sometimes adding everything up by category is illuminating- did you want to be the people who spend 20% on "entertainment"? We don't actually track small categories at all but do a deep dive if our "personal" category gets too big. Personal is just anything that's not an explicit or implicit joint decision. It's where we acknowledge that sometimes our priorities differ and we don't have to discuss every little thing. I hope this is helpful :)