Once you have the MCU and BASIC-52 ROM image, you somehow need to get this image into that chip before you can start having fun. If you already have a programmer that can do that then you are reading the wrong page. If you don't, here are some options for you -

You can have a friend to program the MCU for you. OK for one-off, but not a feasible long-term solution. A better option is to build yourself a programmer. One of a simplest parallel port programmers I've seen is a by M Asim Khan. This is designed for Atmel 89 series controllers, but probably works for others, too.

If this is too complicated for you, switch to AT89S8252 controller, this is serially programmable, in circuit. Other than being ISP, 8252 is a 8052 with 8K of code EEPROM and extra 2K of DATA EEPROM.

Once you have the MCU, you can program EPROMS with BASIC-52, or you can also build a EPROM programmer to ease the task. Here is one design by Andrew J McCubbin, here is another by André Bauer.

All these are simple enough to be a nice weekend project, and won't cost you a fortune. If you have been tinkering for a while, you may even be able to build any of these programmers out of scrap in your junkbox.

Once you have the programmer, download the image (and source code), and program the MCU.

Now that the full documentation is also available here, you can get going right away.