Attach a contract to your invoice if you require your clients to sign before paying the invoice.
Go to projects and open up the project you wish to send an invoice to. Find the invoices section and click "+ Add".
Click "Options" next to the gear icon, under "Invoice details".
Scroll down, and click "Add a contract".
There will now be an option to pick a contract right under the invoice details.
Select which contract you want to send to your client.
After you've attached the contract, continue creating your invoice.
Add all the invoice items. Click the 3 dots next to each item to delete or edit the price, tax rate, title, or description.
Make sure to enable payment methods.
Click "Options" next to the gear icon and "Payment method". You must have payment methods connected in Settings > Payments in order for your invoice to populate.
After you've added payment methods, all that is left to do is to save the invoice. When you save, you'll be prompted to send the invoice. If you do not want to send this invoice yet, you can always do it later.
To send the invoice, open up the project, scroll down to the invoice, and click the 3 dots to the right of the invoice.