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First step is to know what you want to create. Don't look at the rules first, come up with a concept. You need to know where the thing lives, what it does (in a general sense) and how tough you want it to be.
Once you've done that, you can start looking at game specifics. I usually start with a creature type and a CR range. If I know that I want some sort of zombie-thing to challenge my 3rd level party, I'm probably going to make an undead around CR 2 or 3. If I know I want a winged, fire-breathing monster to terrorize 10th level PCs, I might look at choosing the dragon or outsider types.
Next I start with HD, since this affects so much of the rest of the critter. If you plan on it having lots of special abilities (special attacks, SLAs, defenses, immunities, etc.) you'll want to put it at slightly less HD than the table recommends. If you want it to be just a big punching back without anything special, you might have to give it a few more. Depending on how high the target CR is, determines how much leeway you have there. Along with HD, I go ahead and pick a size for my creature.
Next, I assign ability scores. I assume you know how these works. Most monsters that have abilities either have them dependent on CON or CHA, it seems, so keep that in mind when assigning stuff, but do whatever you feel is right.
After that, I just start working down the stat block from top to bottom. There are tables for BAB, saves, skills and all that. Just plug in your numbers and pick stuff you like. Remember to assign a feat for every 2 HD and the appropriate skill points. If your creature has natural attacks, make sure to check the Natural Attacks chart (in the Glossary) to determine damage and primary/secondary status.
Now put in your special abilities (if you have any). Go thematic first and then work on numbers and rules-language. Steal abilities from creatures you like or make up your own.
Finally, compare your final stat block to the suggestions for AC, Attack, Damage, Save DCs, etc. and see if things sit around the target CR. If so, congratulations! If not, go back and tweak some numbers until it looks about right.
As an added bonus, post it in the Homebrew/Suggestions forum and get some second opinions on it. There are lots of very skilled people here on the forums who are more than willing to help out with your homebrew efforts.
That said, was there something in particular that you were wanting to create?
You might look at one of the hydra variants (cryohydra/pyrohydra) and just modify from there, rather than building from scratch.
It'll probably need the aquatic sub-type and a swim speed (which might replace the land speed), switch the breath weapon for electricity instead of fire or cold and viola! You have yourself an electric eeldra.
The hydra also states that it has 1 head per HD, so if CR 6 is too high for your PCs, you can drop the HD to 3, with 3 heads, and probably sit it around CR 4. Obviously, you know what to do to make it higher than CR 6.