Using mirrors, it won't be easy.
Suppose you want to make a planet, the size of Earth habitable that is around √2 times too far away from its star to be in the habitable zone, warm enough. What you'll need to do is increase the amount of light it gets by a factor of 2. (Since the light a planet receives is inverse to the second power of it's distance from said star.
There are two options, have mirrors orbiting the sun closer than this planet and you'll need a total surface of Ap(rm/rp)2 where rm and rp are the distance to the star from the mirrors and planet respectively and Ap is the surface of the planet interpreted as a flat circle (diameter of the planet times pi). I don't think it would be easy (or cheap) to have those mirrors always redirecting that light at the planet though.
A more viable solution seems to have a bunch of mirrors orbiting the planet, possibly in this configuration. 24
In this case the distance of the mirrors to the star would be equal of the distance between the planet and the star, so the above formula evaluates to: Am = Ap, the surface of the mirrors should equal the apparent surface of the planet . For an earth sized planet, this number is about 113 million km2. If we take circular mirrors with a diameter of 11.2 km (which gives them a surface of 100 km2, we'll need 1.13 million of them.
If you allow those mirrors to be essentially side by side in orbit, you need them to be about 1.13 million km away from your planet, which seems doable.
In order to spread the light reflected by these mirrors nicely across the globe, the mirrors would need to be slightly concave. And will obviously need to be at an angle of about 45°
This will of course heat the poles more than the equators (since the mirrors will always cross over the poles) and will also illuminate the night side of the planet, but I'm sure you could solve some of those issues by adjusting the shape of the mirrors.
If you're wondering how bright these mirrors will be, the will in total be as bright as the sun and have a total angular size equal to that of an earth sized object at a distance of 1.13 million km away. This gives it an angular size of roughly 2.7 times that of the sun, which would make it very bright, but not quite as bright as the sun, I imagine it would look like a very thin bright line in the sky.
All in all this seems achievable, especially if the planet isn't far out of the habitable zone, but I can imagine there might be better solutions. 921