help with time capsule

It will back it up when it's plugged in, just turn on Time Machine - make sure the initial backup completes - then leave it on.

The backups will occur hourly, if you want to force an immediate backup (i.e. you want to plug it in & get the backup updated without waiting for the next hourly run) click the Time Machine icon > Back Up Now

 

This program will do what you want and more...

However, once it is plugged in, just leave it. Time Machine is not disruptive at all, and you will be totally backed up when the unthinkable hard drive failure happens.

 

1) Open Time Machine preference panel
2) Turn Time Machine to 'off'
3) Tick the 'Show Time Machine in menu bar' box
4) Run backups manually using 'backup now' from said menu bar item

You might also want to look at the Time Machine Backup Schuduler (linked in the preceding post); the very similar and significantly cheaper TimeMachineScheduler, and TimeMachineEditor.

I've played with all the above apps and ended up going for the fully manual option, because a) I trust myself not to forget, and b) I don't want an external drive on my desktop all the time.