When you set up your budget, put down all the things you know you have to purchase, then add 20% contingency. You aren't a construction pro and some costs are hidden. Or start with your budget, subtract 20% and then set your priorities / estimate the elements from there. If you will DIY, be realistic about how much you can accomplish yourself and where you should get a pro / consider your proposed schedule with real life - time for work, meals, family care, set up and clean up.

General rule of thumb is that things take 2x as long as you think - you have coordination between contractors. Paint colors are worth testing - even if the kind of paint you selected doesn't sell samples, the home stores will and will (closely) color match your paint chip so you can test it before you buy gallons. Some people can tell from a chip but most need a larger sample and to look at it in am, afternoon and artificial light. Lots of mistakes on paint colors add up to lots of time repainting - not the most expensive / difficult problem to fix but very disruptive and disappointing.

Quite often I see people post part of their work installed while they are trying to figure out materials for the next part (backsplash. To the extent that you can (cash flow), organize, sample, decide and (if diy - purchase) EVERY material, appliance or fixture purchase before you begin if you diy. Why? It takes a lot of time and you have more before you start. This can be critical for schedule - even if you think you can just buy it online and have it shipped in a week, it may be backordered or your quantity may take longer. It is more critical for the overall result - many an exhausted DIY'er has settled for what they could find rather than what they wanted / could really fit the budget.

You want to have a cohesive result and consider how all the elements work together color and scale wise - many a choice has been rejected when it arrives and is seen in toto. Contractors can move forward and better estimate the work they have to do for you if you have sample material - though if you are planning to use subcontractors they can often buy material for less than you can / contractors discount - then you need to pay them procurement time too or offer to handle material pickup for a discount if you have the time. If you are contractor shopping, consider whether you want to manage all the subtrades or have a general contractor coordinate. If you work full time, a lead contractor is going to be worth his weight in gold. You can't substitute for experience.

Finally, ESPECIALLY if you are a neat or clean freak, stay out of the construction zone except to leave a trash can, do a cleaning on the weekends or it will drive you mad. Designate a worker bathroom / way for them to get ice /water. Especially for kitchens and baths -set up a substitute kitchen or bath in another bathroom / garage with your refrigerator and a dry sink and microwave and table so you can function.