They can be sown from early spring onwards, germination takes a couple of weeks, upto 3 weeks. Give them temperatures of 16-20C ie. 60-70F and just cover the seeds lightly. It may help if you cover the seed pots or trays too, to help conserve moisture.
After germination, transplant them, and give them good light conditions, though beware of really harsh sunlight, until they've grown well. Also give them some time to adjust to outdoors conditions, before leaving them outdoors permanently, by giving them a few hours each day over 2 or 3 weeks outdoors, in non-frosty conditions.
Depending on where you live, and last freezing dates, if you sow in late Feb you will get germination in March, giving them a couple of months growing time until placing outdoors permanently from sometime in May - subject to your locality.
There are a few different varieties of catnip, so get the type that you most want, or have a mix, as your cats may enjoy some more than others - mine love it all!
Hope this helps. Good luck! Rob