If you have already used your personal bank/ credit card account for personal and business purpose or you have used your business bank/ credit card account for personal and business purpose, you can create an excel sheet/ google doc/ or work on a piece of paper to make a chart for sorting all the expenses. You can sort them under personal expense and make multiple columns for business expenses (example supplies, internet, etc.). If you are using a software for book keeping you can link your bank account to it and categorize your expenses as personal or business.
The best way is to keep a seperate accounts for personal and business use. The business bank and credit card accounts should be under business name. This helps being organized and keeps your income and expenses ready during tax time.
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