The cycle of board game design is make changes, playtest, make changes, playtest. Many games will need a hundred or more playtests.
Your game must be playable two-player. If you thought it was hard getting one playtester, try getting two (or five) at the same time.
If your game won't work properly with two players, don't make that game. This rules out most voting, trading, negotiation, and hidden identity games. You didn't need to wander into those specific genres anyway.
Make sure there's nothing stopping your game working with higher numbers of players, but leave it until late in development to start actually creating the components and structures for those extra players.
Your game should ideally be playable in half an hour.
Your game should be medium-light at most, in terms of game weight. Many people can't properly play, enjoy, or analyse heavy games. Also, as game complexity increases, development time increases exponentially.
No playtester wants to read rules — not even a small rulesheet. They want you to explain the game to them. Just have a reminder card, so players can look up the complicated and hard-to-remember bits, and how the scoring works.
Once you have your online playtesters, you'll need Tabletop Simulator (TTS). That's what almost everyone uses for playtesting online. It costs $20, and simply provides a 3D space containing a table. You can import cards and other bits into the space, and play all kinds of games in there. It's not hard to use.
I use Discord, to chat with people, and we playtest each other's games. Feel free to get on a Discord server, and ask people more about Tabletop Simulator.
I haven't done real-life testing since about 2017.
Also ask your playtesters what their feelings were, after your playtest. It can be hard for them not to give technical details, or analysis.
It takes time to drill this attitude into your playtesters, and then have them be honest about what they actually felt. Most playtesters simply cannot separate analysis from feelings. Most playtesters need encouragement to say "I felt powerless" or "I was frustrated."
When asked what they liked or didn't like, they tell you what would or wouldn't be fun, or tell you about balance. They also feel like they should also offer solutions, which are typically not useful. I have to specifically tell them that I really just want them to complain about things.
This is a very important point: negative feedback is really the only useful feedback. Positive feedback is fantastic, but you can't usually act upon it. Negative feedback is a gold nugget of new insight.
I interrogate playtesters, specifically asking them for negative feedback. I create a space in which negative feedback is welcomed, not awkward. "What part was the least fun?" is a great opener for negative feedback. I'll sometimes add "was it boring, too complicated... too long... not interesting enough?" This specifically asks for criticism, and the criticism doesn't denigrate the whole game, just the least fun bit. It also isn't asking the player to analyse the game — only their own feelings. It's your job to cater to those feelings.
I like to ask the big questions, too.
"Is there enough or too much in the game?"
"Is the game the right length?"
Also, I ask them for ideas for new things.
What's in it for me? That's what you should be considering, from the point of view of your playtesters. Unless you're mutual playtesters, they don't have to help you.
Make them feel valued, and like they're really helping.
Make sure to agree when they're right, or have a good idea. Don't argue when they're wrong, unless you're just correcting a basic fact they're basing feedback on. Just listen.
Tell playtesters you're "interested to hear their opinion" (which you are, even if you only sometimes act upon it), and thank them for their feedback.
Always end a playtest early if the game breaks, or its problems become obvious. Don't make the playtester continue, unless they want to.
Also, it gets easier when your game is good, and playtesters want to play. I'm currently not losing playtesters, like in the past, when I was wasting their time and mine on an unfixable prototype.
Always remember, however, that you're the designer. Playtesters are not capable of doing your job. They're just an opponent, and a source of feedback and ideas, at best. I've done large stretches of productive playtesting with playtesters who provide no useful feedback or input.
You are the main playtester.
This point might sound obvious, like saying "make your game fun", but it's actually a crucial part of my design process. Forgive me if I labour the point.
Be constantly aware of your emotions during playtests.
Normally, we're trying to ignore negative emotions — not monitor them. In day-to-day life, we don't haul a bag of groceries, and analyse ourselves as we do it. We don't think "my hand is sore... my arm is sore... I'm somewhat frustrated..."
It's a strange thing to do, but if you're experiencing a negative emotion while you're playing your game, other players will too.
You're creating an emotional experience here, and you can test it on a human — yourself.
Earlier in the development of Radlands, I noticed that I had more fun when there were less cards on the table. When there were more cards, the game could become a slog, and it was hard to impact the opponent. While the slog was entirely fair, I found it frustrating and boring.