Profit and cash are two different things
You can be profitable and still run out of money. If you send a €5,000 invoice but the client pays in 60 days, you have profit on paper but zero cash in hand. Meanwhile, your own bills are due now. Managing this timing gap is the main task in cashflow management.
Invoice fast and clearly
Send your invoice right after the work is done, within days. Set short, clear payment terms (for example 14 days), and make it easy to pay. You can only collect money after you have sent the invoice.
Set aside VAT and tax immediately
The VAT you charge is the Tax Authority's money, kept on their behalf. Transfer it to a separate account as soon as you receive it, along with your income-tax reserve. Spending tax money is one of the most common, and most painful, first-year mistakes.
Build a buffer
Build a cash buffer for several months of costs. You then have money for late payments, quiet periods and unexpected costs. If you have little cash in reserve, even one slow-paying client can cause you problems.
Watch your timing
Make an overview of when you receive money and when you pay it out. If you must pay large costs before your clients pay you, plan for that gap. Make a monthly forecast of expected income and expenses, so you see problems in advance.
Chase late payers early
Follow up on unpaid invoices quickly. Send a friendly reminder the day after the due date. This is normal and it works. The longer you wait, the harder it is to collect the money.
You can arrange the notary, the KVK registration and the VAT number from abroad. See setting up a BV in the Netherlands.
Related questions
See also Applying for a Microloan as a Starter (Qredits) and Building a Financial Buffer as an Entrepreneur. You can also read about your first year of Dutch taxes.