Bella Sara, as a community, mostly exists in two spaces: Facebook groups, and the Discord server North of North (and also the Reddit page).
Both places require a profile to join. Facebook groups require a moderator to accept your join request, and joining the Discord requires a invite link, which can be procured from Reddit, other neocities BeSa sites, and BeSa community members. Once inside these spaces, you can work on advertising that you are ready to trade.
You should already have made an ISO (In Search Of) sheet with both cards you are looking for and cards you are willing and/or able to trade. These can be posted in the appropriate sites.
Other community members will have these sheets posted as well, or pictures of cards they are looking for. It will take time to look through other people's lists to see if they have something you want and you have something they want. If you've found something, the etiquette is to let them know you're interested. This is done as a request to Direct Message (DM) or Private Message (PM), after which, you can send a request to message them.
Once you've gotten into someone's DMs or PMs, all that is really required is to say "Hi I have [this card] and I'd like to trade for [Other Person's Card]?
If you've reached out, it's up to other person to agree to the trade or not, and vice versa. Nobody but the two people entering into the trade can determine if it is fair. Sites like The Unofficial Bella Sara Database can help you know if it is "fair," but again, fairness is subjective (Disclaimer: I am a volunteer on that site). Plenty of traders, myself included, have their own internal valuations of cards and often trade both down or up in value.
The most common method of "value" is to trade equivalent cards for equivalent cards, ie 1 common card for 1 common card and the like. While this method is useful, it requires some knowledge of which cards are more valuable (or worthless). For example, it isn't a fair trade to swap one Ancient Lights rare for one Baby Bella one. This knowledge can only be gained by doing one's own research, using guides like the database and looking at "sold" listings on Ebay. However, many traders--myself included--do find this approach to be sterile and are often not "trading at value," because if we all did that no trades would occur. I personally tend to be a bit more generous with my trades, especially with new traders, because a) I have a lot of extra cards and b) it's nice to do. The Bella Sara community has always been a kind place to me and I attempt to bring that back into the world.
Some BeSa series are always in demand, but rarity can change in an instant. Two years ago, Herds cards were quite rare, and then someone sold a whole lot of booster boxes on Ebay for cheap and now many collectors have spares to trade, and a whole set runs about 50 USD. Generally, the first four Danish sets, Magical Friends, Baby Bella, Best of Bella Sara, and Winter Festival are all sets with high individual card values. While the market for these flucuates in dollars, they are good "currency" in terms of the trade market, as they are always in high demand.
A note: for many if not most traders, there is no expectation of trading "mint" cards. Many rare Bella Sara cards are not in mint condition and cannot be. Some traders are looking for near-perfect cards, but this has been rare. Some light wear at the edges ("near mint") is par for the course. For most traders, the most important thing is that the artwork is intact. If your cards are bent to creasing, have marks or scratches, or are stained, they are considered damaged and must be valued as such. Some cards may have sharpied writing on the backs, especially if they came from children's collections. This should also be disclosed, but I have had no issues as long as I am forthright with conditions such as these. Many traders do not care about the backs of the cards, but some do. It is always good to ask.
Additionally, and I will certainly restate myself, you can back out of a trade at any point as long as neither of you have mailed cards. You can also refuse a trade if you are at all uncomfortable with either the offer, or the person themselves.
Once you and the other person have agreed to trade, a conversation about shipping may be necessary. Often, most traders will advertise their preferred shipping method prior to any trades, so both parties do not actually talk about this. There are only two ways to mail a card: tracked and untracked (also known as snail smail, regular post, letter mail, etc). If both of you want to pay for shipping, this must be discussed and agreed upon by yourselves. If the trade is through letter mail, there is no expectation that the other person should pay for a stamp. It is, in effect, "free."
Tracked shipping can run anywhere from about 5 USD to 15 USD, and is usually done for cards of high value (for me, this is any singular card worth more than 40 USD), or when there are too many cards to mail in an envelope.
Finally, after the cards and the shipping method have been mutually decided upon, you can ask for the other person's address and give your own.
It is very simple to trade online. What isn't so simple is mailing envelopes, which is what inspired me to make this website in the first place. Go to How to Mail Trades!