Put your mince into a bowl, and then pour in the chopped vegetables.
Add in the salt and pepper.
I use my hands for best effect with this, as I find that a wooden spoon takes too long, and just doesn't allow the peppers and onion to be fully mixed with the mince. It takes a couple of minutes to thoroughly mix the food together until it is in a loosely bound state.
I use a cheap burger press I bought from Lakeland that makes burgers around 180g each, but before that, I used to make mine by hand, simply taking a dollop of the mixture and shaping it into a patty with my hands.
For smaller burgers, you could easily get around 10 from this mix, but I made 5 larger burgers.
Everyone had different requirements for how they like their burgers cooked, ie red in the middle, fully cooked, or almost burned black. I like mine well done, so gauge your own timings for cooking in a moderate oven for as long as you need to.