29 speed - do you mean 27 ? ie. 3 x 9 => 27
In which case you have a 9-speed system so you'll need a 9-speed spare link : 6/7/8 are all same width, 9 is narrower, 10 is narrower still - you just need one the right width for your chain.
Is it Shimano or Campagnolo ? (i.e. the shifters/gear mechs - not the chain itself)
If you have say Shimano 9-speed, then any link that fits 9-speed Shimano will be fine, whether you get SRAM, KMC, whatever.
Any will work with a 9-speed chain.
You need a link extractor tool (aka 'chain breaker') to drive-out the pin from the chain.
The two halves of the link just go through in place of the two pins you've removed, then the two halves simply slot together.
If the chain broke by a pin coming out, you drive-out the next pin along, remove one link from the chain and replace it with your spare link.
You can, as a temporary repair to get you home, shorten the chain by a link or two.
If you shorten by too many, you won't be able to use the big rings front or rear, but you could limp home using just the smaller rings.
If the rear mech is bust/ripped-off, you could re-route the chain direct from one of the rings at the front to one of the cogs on the back, without going through the derailleur, shorten the chain to be the right length, and then limp home WITHOUT TOUCHING THE SHIFTERS
- basically you'll have made your bike into a single-speed, fixed in that one gear
- so it's best to go middle front ring on a triple/small ring on a double to one of the middle cogs on the back : you want a gear you can actually pedal, not one that's too high or low
- and you want the chainline to be straight, i.e. the rear cog to be directly in-line with the front ring, so the chain goes straight not pulled off left to right