Art collections, unlike art sets, are unique because you won't probably find someone else who has the same art pieces put together the same way. When done well, art collections can help you create eye-pleasing home decor. While art is very subjective, people mostly tend to agree on what feels eye-pleasing and what doesn't. It's not about the style of art you want to decorate with, it's about the harmony of the pieces together and the general aesthetics. I will talk about a few points that are good to follow in order to create aesthetic art collections from the pieces you already have and love.

The best art collections don't feel forced together by rules, their only purpose it's just to decorate but are naturally created over time. Those are true collections, not just decorations. Use art that matter to you, that you have for reasons - it's been given to you, painted for you, it has a meaningful history or you got it because you really liked it, and just add something new to it if you feel it would go well together and it would add the overall art experience. So follow the tips below but don't feel obligated to curate your art collection the exact same way.

Just a word: A collection is usually made of more pieces than a set. Art sets are often very closely related, they have the same style, same set of colours. Generally, there are two to four pieces in a set, usually not more.
Sets are often sold already curated, so you don't have to spend time choosing the right ones that will go well together, just decide the sizes and how to position them on your wall or a shelf.

Placement and sizes can vary and as well as choosing the right artwork, the positioning in the collection is also an enjoyable and artistic experience. It should feel fun. There are no given rules on what must be done and how. Just tips, because beauty is in the eye of the beholder, indeed.

So when you're on the look for pieces that would make a nice collection - perhaps together with pieces you already have, here's what to look for:
Choose the art piece that will be the main focus of the collection. It's usually rather a large piece but it doesn't have to necessarily be the largest one of the collection. This main piece will be the heart of your collection, you may want to place it centrally (although that doesn't mean it must be precisely in the middle of the intended area where you want your collection to be). Other art pieces should follow the colour aesthetic and more or less art style and general feel.
A tip: The main piece could also be a set. For example a set of two prints that complement each other and create a very cohesive set. Other pieces will be complementing them more loosely to create a collection.

Now when you'll be deciding what goes where I've already mentioned the tip to place the main art piece centrally. How about the rest?
If you want to have a more natural collection or a more geometrical one, that's up to you and your preferences.
A naturally curated collection doesn't have exact rules on artwork placement, but it's good to keep sensible spaces across all pieces. Smaller pieces could have smaller gaps, while larger frames may need a bit more space around them. What you may want to avoid is a feeling of emptiness (too large gaps between the wall art pieces) or a stuffed feeling (artwork too cramped together).
A geometrical collection may be easier to create once you decide on the way you want to place the art. For example, placing six frames in two rows and three columns, etc. There are many ways how to use geometry when hanging art pieces (hexagonal shapes, circular shapes), even when mixing up smaller and larger frames - usually the sizing would have some geometry to it too. But when you go the route of keeping things in order, then it's more important to measure and place everything really well or the geometrical aesthetic would be ruined to the naked eye. How precise you should be, depends on the size of the art pieces as well as the whole collection. The larger the artwork as well as the number of pieces, the easier would a misaligned piece be forgiven when looking at it with a naked eye.

Quick tips: 
> A cohesive colour palette works well, you can go a bit wild but keep it related to your main piece (or centre set).
> If you have a secondary piece of large importance, better to not keep it too close to the main artwork.
> You can mix up sizes as well as vertical and horizontal orientation to create beautiful wall art collections.
> Keep the size and general orientation of the collection in relation to the wall where you're placing it. A large long rectangular wall would be better suited for a collection spread out more along the width of the wall than a collection placed into a squared or round shape.

When you follow these tips, creating a beautiful wall art collection should be an easy and enjoyable task. You can also always ask your friends and family for opinions but there's no right or wrong in the world of art. If it makes you happy and you enjoy looking at it, then it's certainly perfect.