For both sites: Take good pictures. Multiple ones from different angles. Write a good decription. Personalize it a bit (people like that).
For Ebay: Figure out how you're going to ship it (packaging needed, size and weight of package, delivery speed, delivery company, etc). Ebay will calculate (and charge the buyer) the approximate postage depending on your buyer's location and your specified size and weight. Then just print out the label and attach.
Craigslist: Cash only. No seriously, cash only. Meet in person only. No paypal, no shipping. Paypal is a scam on CL. Every. Time. Shipping is a scam on CL. Every. Time. Cannot stress this enough. Price your item for a bit more than you want to sell it for. People like to negotiate on CL, so allowing them to "talk you down" makes them feel good about the purchase. Try to talk to them on the phone. Not just texts and emails. Talking on the phone increases the likilihood that they'll actually show up. Make this exact note in your ad "If this ad is still up, the item is still available." This avoids multiple emails just asking if the item is still available. Just make sure to take your ad down once you've agreed with a buyer. Of course if they flake, you'll need to repost again.
CL is more of a hassle, but it's local, and you'll get more money. Ebay is easier, but they take a bunch of fees and in addition you can lose some to postage and packing materials. It's also limited in item size, whereas CL you can sell everything.
I personally prefer CL, but others like EBay better. Try both and see for yourself.