The machine would not actually travel through time in a linear fashion, rather it would create an envelope that would cause a warp in the space / time continuum. This is already theoretically possible. The tricky part is how much energy would be required to produce such an envelope.
We are talking the energy produced by a galaxy would have to be harnessed. Perhaps we could master that much power some day. The next tricky part would be controlling the point where and when the travler would emerge from this “envelope”.

The same way a teleportation machine would be made.
You take the atoms of the something you want to teleport, make them store into the machine, carried around by telephone wires or something, and BOOM! The atoms come out the other end.
But for a time machine, it would have to travel over the course of time instead of space. So if a teleporter is made, fix it to whisk you over the course of time instead of over a distance. My guess as to how it would work is that it would break open a distortion in the fabric of time and jump in, landing in a different time as a result.

My theory is that a time machine will most probably never be built. The future simply doesn’t exist and the past is just that – the past. Time just keeps creeping along – and in only one direction.
The idea of time travel is intriguing – but unfortunately is destined only for the realm of Sci Fi.