When you receive crypto
Crypto you receive for content, sponsorship or services counts as revenue. The value is the euro market value on the day you receive it. The same barter rule applies to free products. Send an invoice in euros and note the coin amount and the exchange rate. VAT applies to the service as usual, whatever the means of payment.
Price changes after you receive it
Inside the BV, the crypto counts as a business asset. Later price changes count toward the profit when you convert. If you convert at a gain, the gain is profit, taxed at 19% or 25.8%. If you convert at a loss, the loss is deductible. Under careful Dutch accounting rules, you value the crypto at cost or at the lower market value. You can therefore usually wait to pay tax on a gain until you sell, while you may take a loss on paper earlier. Privately, it works differently. In box 3, you pay tax on a notional yield on the value on 1 January. If your real return, including price changes, is lower, you can show that and pay tax on the real return instead. The same comparison as in investing through your BV or privately applies to crypto.
Convert part of it on the day you receive it
You owe the tax on crypto revenue in euros, whatever happens to the coin's price later. On the day you receive the payment, convert the tax part, including the VAT, to euros. You can keep the rest in crypto if you prefer. If you skip this step, you may owe euros on a value that has since halved.
Banks and records
Dutch banks accept crypto-linked business payments most smoothly when the administration is airtight: exchange statements, wallet records, per-receipt euro values, a registered exchange in between. The same clean records that satisfy the bank also satisfy the inspector. For how this combines with the rest of the setup, see start here.