Step 1: Tools and Supplies
Standard size hacksaw (A coping saw may work fine as well)
Carbide saw blade (Glass is harder than steel, so a regular blade won't work)
Leather work gloves
source of water
At least three different pieces of sand paper of various grit (I used 220, 100, and 60)
Roll of masking tape
A base to set your bottle on while you saw. I used a 9X9 Casserole dish turned upside-down.
Home Made Jig
While I would like for everyone to use my instructable, you may skip it if you can get your hands on a Tile saw such as this one:
Still be safe, wear protective gear, and use step four to help smooth out the edges.
Step 2: Getting Ready
I've used both round and square bottles, and they all work so far (I have not bothered with flask shaped bottles). However, thinner bottles (beer, zima) are more fragile and tend to break sooner than liquor bottles, so you 'll probably spend a little more time sanding/grinding down the bumps and edges, but more on that later.
PREPARING YOUR BOTTLE
First, think about where you want to cut your bottle (keep it horizontal). This is one of those 'measure twice cut once' situations.
Draw a straight line along the center of the tape, then wrap it around the bottle. It's easier to line up two ends of the line when applying the tape than trying to draw a straight line on the bottle. In case you're thinking about just 'winging it' without using masking tape, don't. When you start sawing your bottle the tape will help your saw from slipping off your mark.
PREPARING YOUR WORK SPACE
A side-effect of sawing glass is glass dust. Very Dangerous! You don't want to breath it in, get it on your skin, or get it in your eyes. It's like poison. Instead of wearing a mask and other protection, having a steady trickle of water on your bottle while your sawing will remove the glass dust.
I used a sink, but you can probably rig up a water hose if you want to work outside. If you do work over a sink, setting your bottle on a base in the sink will give your more freedom of movement. An upside-down wire dish rack might make a good base.
Step 3: Getting Started
Start sawing by making very short strokes until you got a groove, then go to town. Sawing glass is about quantity of strokes, not the quality. Putting to much weight into your strokes could ruin (break) your project; it's much better to saw quickly and lightly. You knew this wasn't going to be a five minute project; don't get hasty.
The trick to sawing bottles is not to try to saw all the way through. Because glass is so very brittle, it doesn't absorb the shock of sawing very well. Once you break through the glass, the saw starts catching on the glass. Also, glass doesn't flex like wood or metal does when you cut half-way into it, so it breaks before you get to the end.
I keep putting off making a jig, but two perpendicular pieces of wood with a slot for your saw would be very helpful, just strap the bottle to it.
So instead of sawing all the way through it you'll make three cuts into the bottle, leaving little pillars. These pillars keep the sides of the bottle from trying to flex, thus snapping the rest of the bottle. Once your saw starts catching, rotate the bottle forward a bit and saw only on one edge of the hole until the hole is about a third of the bottle.
Once you've made your three cuts, gently begin sawing the pillars. At least one of them will break before it's over, but this will decrease the area you have to sand down. Using a jig should delay the final breaking.
Step 4: Finnishing
If you have a dremel, this process may take less time. Recently, I started using a stone grinding head. Now, the main problem with using a high speed power tool is that the friction had heat the glass very quickly, prompting it to chip. To combat this problem I like to submerge the glass edge just under the surface of a pot of water and dip just head of the dremel in the water while sanding. This will help regulate the heat while keeping glass dust out of the air. Keep using dripping water to control glass dust.
BE SAFE. Continue to wear gloves and goggles. Once your glass is the shape you want it, use the higher grit sand papers to bevel, or round down, the inside and outside edges to your personal satisfaction. You can never go too high or too smooth, just don't expect the edge to shine like all of your store-bought glasses. Sawing the glass will have already prevented a lot of sharp edges, but beveling removes hidden sharp spots.
If your bottle had no labels, then you're done! If not, then try to remove them as best you can. Soaking them in water helps, but sometimes you're left with glue on the sides of the glass. Bottles with plastic labels aren't as bad about leaving glue. I haven't had any luck with "Goo Gone." I've gotten the most success from applying rubbing alcohol and scrubbing with a steal wire brush., but mineral spirits and lacquer thinner would work pretty well, as coarse steel wool would be just as good, if not better than, a steel wire brush.