Open WPS Writer or Spreadsheets. Look at the top right corner. You will likely see a small icon that looks like a User Avatar or a "Sign In" button. Alternatively, click the Menu (three horizontal lines or the "WPS" logo in the top left corner).
If the cost is still too high, remember that only PDF conversion and OCR are truly unique to WPS Premium. For strictly word processing and spreadsheets without ads, LibreOffice is 100% free and open source (though it lacks the modern UI of WPS).
WPS offers a "Premium Family" key that covers 3 devices and up to 5 family members. This is cheaper per person than individual keys.
WPS Office Mobile (Android/iOS) offers a subscription via Google Play or Apple Store. If you subscribe via mobile, you often get a key to unlock the PC version as well.