The picture shows some sample UPSs and an example of the gel cell from one of them. The UPSs come in various capacities and, although you can boost the capacity, the output power is fixed. When you start out, make sure that the UPS you're going to modify will provide the volt-amps and power that you need. Also note that the volt-amp rating is higher than the power rating. The difference is because AC powered devices have a power factor. Check online for more info about this. Another, similar Instructable, also warns against trying to max out the capacity of your UPS because some use transformers that will run continuously at the rated output. It really depends upon the quality of the UPS, but plan to run at not more than about 75% of rated output capacity.
Another thing to consider is whether or not your scrap UPS has AVR or Automatic Voltage Regulation. You'll want this if you can find it.
Step 1: The Guts
When you open the UPS box, you'll usually find either one or two gel cell lead-acid batteries. You're going to replace it / them.
What you'll need:
1 scrap UPS - but it needs to be a WORKING UPS!
4 feet of #10 copper wire (2 feet of red and 2 feet of black if you can)
You may need to adjust this length, but keep it as short as you can tolerate.
4 spade lugs (two mating pair
2 ring lugs for single battery or 4 if you have two in the UPS
1 or 2 marine, deep-discharge batteries (85 - 120 AH capacity)
A drill with bit set
Crimper for the lugs
Sandpaper or a small file to smooth out holes in plastic
Some notes on the parts: look at the pricing of the batteries and get the capacity that gives you lowest cost per amp-hour. Sometimes the 110-140 AH batteries are only a little more than the 70-90 AH ones. Check warehouse clubs for good pricing. Sometimes you can reclaim the core charge with an old motorcycle or car battery you have lying around. Make sure that what you get is a deep-discharge type battery. Some batteries are labeled as marine, but not deep-discharge.
Make sure that your battery has adapter studs that clamp to the lead posts. Then, get ring lugs from your hardware store, electronics store, or online store that will fit over the threaded studs and also accept #10 wire.
You can either get two pair of mating spade lug connectors, or get one set that are the mating connectors to the ones that come from the UPS. In the picture, these are the black and red wires that connect to the batteries.
Note the blue wire. The batteries are in series in this UPS, so it requires 24 volts (black terminal connected to red terminal). Do not try to power it with one battery if it comes with two! Get two more ring lugs if you need two batteries.