In their exploration of a balance bike kids will usually stand over the bike frame and walk as thier first step. Don't force them to sit - this can often lead to resistance. This may be their longest learning stage so don't push them onto the next stage until they are full comfortable standing and walking. Follow your childs lead and provide them with lots of opportunities to practice.
In time your kid will sit on the balance bike seat. But most likely continue with walking. It is at this stage the realisation that if they want to pick up speed they can do so kicks in, and the real fun begins! Bring your child to the park with other children so they can practice together, gently encouraging them to move a little quicker if they feel ready. If frustration starts to creep in then it might be worth sticking to balance bike adventures on thier own rather than around other kids. Again, let your child lead the way.
Once your kid is sitting on the balance bike and moving along at a quicker speed you can introduce them to the footrest and brake if you have not already done so. Encouraging them to lift their feet off so they can glide along with good balance, put their feet up on the footrest and when needed pull on the rear brake to come to a safe stop. It is advised that you take your kid to open areas where they have lots of space to practice, and educate them on where they can or cannot ride.
The last stage of learning to ride a balance bike is certainly the most exciting and thrilling stage of the process, and possibly the most tiring for you as you try to keep up! It is at this stage that you can take your kid to places where they can go over bumps, hills, more challening surface, possibly use the footrest to stand themself up, and maybe even enter competitions!
The best piece of advice is don't rush your kid. This process should be fun and joyful.