I recommend this UART to USB converter, 3,3 V 5,5 V FT232RL FT232 FTDI USB Zu TTL Serielle Adapter Modul Mini USB Port Pro Mini USB ZU 232 USB Zu TTL Diymore Für Arduino - AliExpress 502
UART-USB-TXD —> Spartan 3 Lite UART RX (Yellow)
UART-USB-RXI —>Spartan 3 Lite UART TX (Orange)
UART-USB-GND —>Ground to where Spartan 3 Lite Electronics Ground (Black) is grounded
You can use any UART to USB converter you wish, ideally you want one based on the FTDI chipset as they are the most common and most operating systems like Windows and Android already have the driver built in so no additional driver install is required
For instructions on how to send serial commands using the UART to USB converter, read section 7 of The Spartan 3 v2 User Manual, Software and Documentation | 14Point7. That manual is for Spartan 3 and not Spartan 3 Lite but the sections regarding USB communications applies.