Cot to Bed transition

[Aphy]

Hi Everyone, we are reaching that stage where we have to start considering moving our 2 year old over onto a big boy bed. We are going to start ttc a sibling soon so would rather start with the transition before that happens. We will be moving him to a normal 3/4 size bed with those portable railings on the sides to prevent him falling off. My biggest concern though is how to keep him in the bed? Currently, we put him down in his cot while he is still awake and he eventually settles himself to sleep about 30min later. If we did this when he is in a proper bed, he could and will stand up etc and risk falling over the barrier. How does a person do it? He can then climb off the bed anytime he likes. Please can other parents tell me how you got it right...