Ruth,
In short buy one. Do not think about making one.
One route is to find a "teleconverter" for a 35mm camera, and dismantle that. These are essentially 2X or 3X barlows, junk ones can be found on eBay for a few $. They should work well enough with a telescope though your main problem will be to remount this thing to fit a telescope and take an eyepiece behind it. A good-quality 4-element teleconverter however will cost as much as astronomical one to suit a telescope, so frankly you won't save anything by doing this.
Another way would be to dismantle junked 35mm SLR camera lenses in the hope of finding a reasonable negative achromatic doublet that happens to work passably as a Barlow. However your chances of finding one that works well enough in a telescope are are slim to nothing.
The long story... Buy this set of 3 volumes
http://www.willbell.com/tm/tm7.htm and read carefully, several times until you are sure you can do this.
They explain (among other things) how to design and make lenses using home-made equipment.
However, before that you need to be reasonably good at maths, good enough to be sure you can design the thing and get it right before you waste your time making one. Then to choose suitable glass types, design the lenses, buy the glass and the grinding and polishing materials.
Then there is the small matter of some tools, and making a grinding spindle or machine, and your labour (time) to make the lenses, all of this is neither cheap nor quick.
Even if you have managed to make a small negative doublet, you need to center and trim the glass elements and mount it, you will need a machinist with a lathe to do that.
Lastly, virtually all modern optics have antireflection coatings. For a one-off like this it can be done - there are companies in Sydney that will do it - however the cost for a one-off will be several times the cost of buying a good barlow.
At the very least you will realise what really goes into making good optics, and why cheap Chinese optics are an incredible bargain compared to what was available 20 years ago.