BudgetSheet is an add-on for Google Sheets, so the first step is to ensure you have a Google account and a Google Sheet open. You can easily create a new Google Spreadsheet if needed.
Once you have a new Spreadsheet open, click on the top menu item:
Add-ons > Get add-ons.
Once clicked, the G Suite Marketplace add-ons store dialog will open. Click into the search box and search for "BudgetSheet"
If you are a domain administrator and want to install it on your own account only, select the "Individual Install" option. If you have an individual account, just click the "Install" option.
After installation, head to BudgetSheet Setup