Space Simulator
1.0.9
space flight simulator

One of the most useful concepts in orbital mechanics is the Hohmann Transfer Orbit. The Hohmann Transfer is the most common way to get a spacecraft to a different orbit. You can use this technique to increase or decrease your orbital altitude, or change the shape of your orbit.
>The most important point to note is that the thrust you apply at any point in the orbit will affect the orbital altitude at the opposite side.
Increasing DeltaV will increase the orbital altitude at the opposite side and decreasing the DeltaV will decrease the orbital altitude at the opposite point of the orbit.
The below example is a custom mission of the Shuttle Orion orbiting Earth. In this example the orbit has already been made coplanar for visual clarity, although this is not necessary. You can circularize and coplanar in any order.
Here we have an orbit with eccentricty of 0.345. A perfectly circular orbit will have eccentricity 0.
There are no ways to circularize this orbit. One is to apply thrust at the farthest point to raise the altitude to the same distance on the opposite side, and the other to brake at the closest point to reduce the altitude of the orbit on the other side. In this example we will do the former.
In the "Orbit" panel, increase DeltaT until your shuttle is at the apogee  the point on the orbit where you are the farthest away from the Earth. Hit "Select" to put your shuttle in that point.
Once at the apogee, increase DeltaV until the eccentricity is as close to 0 as possible. How close to 0 you can get depends on how close you managed to get to the apogee. You will see the new orbit grow and circularize.
Once you've gotten the minimum eccentricity, hit "Select" to apply the changes and you will have your new circularized orbit.