My understanding of this is that
1. you choose your term structure model (each of them has pros and cons, mainly ability to fit the real curve, possibility of negative interest rates occur, calculation complexity)
2. estimate the parameters based on hypothetical assumptions or using the real curve
3. simulate curves (through a Monte Carlo simulation or binomial lattice, depending on your model)
4. price your bond cash flows on each calculated curve and take the average value as the estimated price.