When I want to make a LiveJournal community, I first think of a name and try to see if there is a journal with that name by adding the name to the end of this url address;
If it takes you to a page that says, Malformed username. either your username has characters that are not recognized or it is too long. I only use lower case letters, an underscore or numbers.
Secondly, you'll need a LiveJournal account for the community and one to maintain it. So if you already have your own LiveJournal, login and see if you have any codes to make your community account. If you have never been to that page before, you'll need to generate your codes.
Once they are generated you'll need to go back to the list of codes and press the link that says use under where it says Redeemed by. That will take you to the page that creates a journal.
Enter the username for the community in the proper text box exactly the way you wanted it, remember you only get one chance at it. Then fill in all the info it askes for; make sure your email address is correct also or you will not be able to confirm the account. Once you have everything correct, press the Proceed button once.
Wait for a few minutes for the conformation email to get to you and press the conformation link, it should be the top link.
Now you are ready to transform it into a community.
Go to the community creation page and fill it out exactly as it says!!! Your personal username and password should be placed in the top two text boxes and the community username and password go into the text boxes right below, don't mess it up or you will be knocking yourself out trying to get your journal to become your personal journal again!!!
The Open Membership is the easiest way to go, that way people can just join with no action on your part. If you choose the Closed Membership you would need to know how to use the console to add members. The console is very tricky, I would suggest to the people that aren't used to html coding or the like to not choose that option.
The next choice, is choosing who can post to your community. Basically the same arguments apply, as the previous paragraph, if you don't have coding skills, poke the All Members radio poker hole thingy. :P Press the Creat Community button and you have your community.
To help yourself in making your community a success, you may want to join, community_promo to advertise it, (Make sure you read the profile completely!!) and join maintainers, (maintainers is a good community to ask questions about running your community.)
Also a successful community will need it's interests packed with as many keywords as you can think of, that pertain to it's subject matter. You'll also need a descriptive bio to inform people of your community rules, (if any,) and exactly what your community is all about.
I hope this helps; feel free to comment with any additions or to correct any mistakes I may have made.
Have fun communing!