A spending baseline shows where your money went before you try to change the plan. Start with a period long enough to include ordinary bills and everyday purchases.
Use statements and receipts to group spending into housing, bills, transport, food and other categories that make sense for you. Count a card purchase once when you track the purchase and its later payment. Keep irregular costs separate so a large annual bill does not distort an ordinary month.
Compare the total with the income you received over the same period. Use what you learn to decide which categories deserve closer attention next month.
Further reading: CFPB monthly budget worksheet.