Here’s a super-brief guide to getting to grips with your titles:
1. First things First
The first word in your title is most strongly weighted on Etsy, and then the second is the second most important etc., so bearing this in mind try to choose the most important and descriptive word/s to go at the beginning.
Avoid using really broad terms such as “large” or “set of four” at the beginning of your title – they’re just not descriptive enough. Likewise, don’t use anything too niche as no-one will find your items!
Sounds obvious, but don't use the fancy titles which you have given to your artwork - as unless someone has already seen the art they won't know to type in "Deathly Days no 7"; you'll be much better using a term such as "Green and Red Skulls Art Print".
2. Use the Space
You have 140 characters – so use them! You can use little searchable phrases and split them up using punctuation (I use hyphens as I find them more visually appealing, easier to read, and it’s what Etsy have recommended to me). Think of the various phrases someone's likely to type in when looking for a product like yours.
So, for example, for a Silver Wedding Anniversary card I would use a title along these lines:
Silver Anniversary Card – Personalised 25th Anniversary Card – Twenty-fifth Anniversary Card – Personalized 25th Wedding – Silver Wedding Card
I stick to UK English in the first instance, but using American spellings can also help. Etsy do recognise both UK and US spellings for common words, in their search, but for less common words/terms it's good to use both.
3. Finding the Words
The words you use for the title and the keywords can and should be influenced by what you get if you start typing into the Etsy search bar. This drop down will show you the most commonly searched phrases on Etsy. Bear in mind that you might not want to use all of these, you may wish to go a little more niche, but this can be a great starting point.
You can also ask friends or customers, pose it as a question on social media (“what would you call this?” - great for some interaction and reach) or use an online thesaurus. It's so easy to overlook words or phrases that people might commonly use when looking at your product, specially when you are so close to the object yourself.
Please bear in mind this advice is based on my own experience, and is current as of early August 2017; the Etsy algorithm is constantly changing, but I’m confident that Etsy will publicise any changes to their search well in advance of them being implemented.