The STC15Fx chips use a simple TTL/CMOS async programming interface that is suited to the common USB-RS232(TTL) adapters. This article describes a low cost programmer that also allows the programming application to Vcc (so initiating the boot loader automatically).
Above is the programmer ($2.50 on eBay) and a small adapter that plugs into the top row of the 2×5 header on the programmer.
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.