The Bootloader firmware resides in PIC Flash memory.

You load it into the PIC Flash using the Simple ISCP programmer, or any other program use use. This is a one off programing of the PIC (until you erase the chip)

There after, you load your programs into the pic using RS232 via the USART pins of the pic.

Not sure on the exact details of how it works, but the bootloader in the pic erases or overwrites the existing program with the new program.

In part yes it is a 2 step process, but only the first time you load the bootloader firmware.

You need to use a MAX232 style chip to load your programs (or a USB converter with RS232 outputs).

Get a MAX232 chip, half a dozen 10uF Caps and some V Board.

I use 3 wires from the serial port, TX, RX GND - connected to a MAX232 chip, in turn connected to the PIC.

See my last post about the software for loading the Bootloader Firmware.

happy days.
J