While it is unknown exactly how the various machines work, they all share the common trait of needing green light to function.  Most time machines are capable of n-targeting, allowing a traveller to choose where and when to enter the timeline.
The time machine that Kendra makes with parts in the garage works by pumping electrical energy into the shard of time they had from the tunnel. This electrical energy increases entropy, which in turn allows them to 'tune' the needle similar to a fox-radio.
Designs and N-Functions
Time machines come in various designs, with basic categories ranging from n-instant to n-continuous. Some machines are n-variable. The N-Value is important when dealing with the PTP, as well as when using multiple machines at once.
Time machines generate lots of Paradox, which can be output into a paradox sink, vented into a different time or grounded into the earth. Paradox is the universe's way of retroactively fixing retcons caused by the simple fact of moving air, and existing outside of your PTP. Not handling the paradox generated by a time machine, or handling it inadequately, may cause brain damage to the time traveller as the brain unsuccessfully tries to reconcile contradictory / paradoxical memories with each other, leading to seizures and possibly death.
Smaller time machines are generally weaker, and some require additional power to function. The more power is input to a time machine, the longer the time traveller can stay outside his or her PTP and/or make changes to the past. Conversely, the more changes are made to the timeline during a trip, the faster energy is consumed to compensate. Once power is exhausted, the paradox winds will force the time traveller back to their PTP.
N-continuous time machines can be used as a form of teleportation, by choosing a point just behind your PTP on the timeline. When you use an n-continuous machine and catch up to your PTP, the jump is rendered complete, and you cannot return to your previous location. This is because you have not yet experienced this time yet, unless one had already locked events ahead of their PTP.
Time machines can also be used to teleport in the opposite direction, by traveling forward using an n-instant machine. Because their PTP is still in the past, and n-instant does not move your PTP, they will return backwards via the paradox winds when the machine is turned off or the paradox sink is overloaded.