The STC15Fx chips use a simple TTL/CMOS async programming interface that is suited to the common USB-RS232(TTL) adapters, some of which are less than A$2 on eBay (CH341 chip).
Above, the completed adapter. DIP-28 are located carefully so that the pins 10-18 are in the socket, the same connections are used for both chip sizes for STC15F104E and STC15F204E.
Above, the copper side of the adapter.
The 6 pin female header accepts a USB-RS232 adapter (break out board style or cable) with the common Arduino pinout. The USB-RS232 adapter powers the chip being programmed, and it needs to be a 5V adapter.
Alternatively one of the little MAX232 adapter boards could be used with a physical RS232 port, but power will be required.
A better idea is one of these adapters for a couple of dollars on eBay.
The adapter above will automatically control the 5V to trigger the bootloader, so no need for the switch shown on the veroboard above. (If you buy, inspect the description and the pics (all of them) carefully, there are other things that looks similar.
To use it, load a chip and set the STC software up for the operation. Press the operation button then quickly press and hold the switch on the adapter until the software notifies the download is complete.