So, budget first, then guest list and then venue. Of course your guest count can (and will) change from the first version of it but you also don't want to have a list with 120 guests if you end up booking a venue first that only holds 100. Trust me, you can't squeeze that many more people in... legally, the venue or fire department could shut you down which would really ruin the night, but also, your guests will be miserable if they are shoved in like sardines!
A little more about the budget... while the spreadsheets you can find online are awesome to help figure out approximately how much you should spend on each item please remember that these are generic sheets and not every category will fit your specific needs or wants. I always suggest to clients to choose the top three things that are important to them for their wedding and work from there. Maybe the food, and open bar and the DJ are the big things to you. Work your budget so that you can get what you want in those areas and then fill in the rest. If your dream look or theme is a ton of candles rather than flowers then what it says on the spreadsheet in regards to flowers will probably be too much. You can then move that money to the decor portion to fit in the extra candles you will need. Basically, don't let a spreadsheet dictate your wedding! I always want my clients to stay on budget (it's one of the best ways to stay stress free) but play around with where the money will go until it suits you best.
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Happy planning, everyone!