I've re-sized both ppc & Intel Mac volumes for Linux / OSX double-booting. I wouldn't go to ehow for anything other than the ads, that's really all they're good at. Boot Camp is good to use first if you want to triple boot Linux/Mac OSX/Windows > install Windows first, Linux last, but for OSx/Linux double-boot best to:
1) Install rEFIt on Intel Mac http://refit.sourceforge.net/ and check that it works on restart (it also needs a reset after 1st Linux boot, read up so you know what to expect)
2) Back up your Mac volume on a bootable volume and boot from it to check its good before re-sizing or be prepared to start over from your OSX installer minus whatever you use to have on your Mac (Carbon Copy Cloner http://www.bombich.com/ is excellent for the job and offers 30 days free fully functional trial)
3) use disk utility in Mac > select your "xxx GB..." hard drive (not your named volume) in the sidebar > select Partition option and you'll see a "single" partition > select and hit '+'or scroll down from 'Current' for number of partitions you want ie. 2 or more. Use the partitions as resizable windows to proportions you want. (On ppc volumes Linux likes to be before your Mac volume which means you'll have to clone, boot to your clone, partition then restore Mac on the last partition) but for an Intel Mac volume you can just resize down without destroying your Mac install. Give yourself at least 10-20 GB for a modern Linux desktop(s) system (its harder to expand after install so try to figure out what proportions work for your situation and needs before partitioning). Bare bones command line only Linux can exist on 2 GB or so but you want at least one desktop and there is a lot of open-source software you'll want space for. Select either 2 or more partitions, Disk Utility won't let you re-size down below what presently exists on your Mac OSX but remember your Mac needs 10 GB extra just to operate your RAM these days. Linux on install will have to create a small boot volume that is too small for Disk Utility to make without wasting space -don't worry about that now (at that time you can delete and re-size whatever partitions you make now in Disk Utility - all except the Mac OSX volume you shrink down to now (actually if you're not careful it will chop that up too but you have a backup, right?) so either think about what size you want to allow for swap, /home and /(root) (/var and more if you want - check the Linux distribution for advice). Make all of your Linux volumes now (they're more placeholders than finalized anyway) or just make 2 partitions; ie. your downsized Mac and one for Linux to divide up and reformat on install.
4) When you're happy with the layout considering what you want to do with your HD select 'apply'. (Its quick.)
5) Linux should recognize the Mac volume while booted into your Live CD/USB installer and will offer both guided or manual partitioning options. If you don't know what you're doing just allow Linux to choose but be careful - Linux will offer to erase your Mac volume if you let it 'choose largest partition'. If you manually partition (in Linux install) it will force you to create the necessary boot volume, a partition for / and proper format (use ext4 for your Linux partitions unless you have a good reason not to). The installer will make you go back and reconfigure until you've chosen options that will install. And if you accidentally erase & reformat the whole drive you've got your bootable Mac backup, right?