Well most likely you can't just go and do it like it's a force of will thing. If you block hard and can't get words out, you're not gonna do it. So instead you should make baby steps, however that may look to you. But seriously, start easy with things you can do or things just outside your comfort zone. Aside from that, see if you can find a good stuttering specialist slp. Blocking is a learned behavior and something you can work to improve. You need to learn to replace blocking with getting sound in on those sounds