Oh heck, thats easy. First you invent a nonexistent problem like... global warming for example. Then you get a cabal of associates to pass themselves off as scientists and experts to back up your claims. Then you convince everyone it is their fault the nonexistent problem is happening in the first place and begin making dire predictions that they will all soon be dead and it's all their fault. You let that simmer for a few years, all the while continuing to make dire predictions and after a while nobody will even care that none have come true. As the saying goes, tell a lie often enough and it takes on a truth of it's own. Finally, when panic levels have hit fever pitch over you nonexistent problem, you tell everyone you have the solution that will save them all, all they have to do is follow you. Then they will give up their freedom willingly and you can do whatever you want to them. Of course, there will be few dissenters and non-believers. At first you will discredit them by questioning their intelligence when they do not believe in your non existent problem. Your cabal of associates will be helpful here. Then once you have most of the people following you they will shout down dissenters themselves. Once you have the power, you just kill them.
Wasn't that easy?