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Blender can get extremely complex if you're not used to similar programs. I highly recommend watching this tutorial if you've never seen Blender or worked with it before:
Start Blender and load your level file. First you should get used to Blender's viewport controls. For this, it is wise to create an object to focus on, first. I can't tell you everything here so you will need to use your intuition a bit. Blender has a very steep learning curve. I recommend looking up some basic Blender tutorials if you're having trouble with this step.
Getting some things clear: (You don't need to understand this right away, use it as a reference at a later point)
You can see a red-white circle in the view port: It's the 3D cursor. This is where new objects will be created. To center it, press CTRL + C. To add an object, press SHIFT A and select Plane. A new plane object should show up:
Try out the following keys:
|CTRL + LMB and then drag||Lasso selection|
|SHIFT + B||Square selection|
|RMB||Select object. Selected objects will be outlined in orange|
|Numpad Period||Focus on selected object|
|MMB||Pan around focused object|
|Scroll||Scoll in and out, direction depends on your selected object|
|SHIFT + MMB and drag around||Drag around the view|
There are many more hotkeys and key combinations. However, for the basics, it's well enough.
One object is not enough. Often it's wise to split your track into multiple parts, or you just want to create an object that is not fixed to the main road. For this, you can either create new objects or duplicate your existing ones. Try out the following actions:
|SHIFT + A||Open add menu|
|SHIFT + D||Create duplicate of an object|
|ALT + D||Create duplicate of an object (Geometry will be shared. If you edit one object, the duplicates will change, too.)|
|SHIFT + RMB||Select multiple objects (main selection bright orange, all other objects dark orange)|
|DELETE or X||Delete an object|
|S||Scale an object|
|G||Grab and move an object|
|R||Rotate an object|
Now you can move, rotate and scale objects. You can also do all of these actions along a specific axis. For example, if you wish to scale an object only in height, press S and after that, press Z. This way you can select the axis. If you only want want to scale it on the X and Y axis, press S and then SHIFT + the axis you don't want. Then move your mouse and click the LMB or ENTER to confirm, or ESC to dismiss the tool.
Be aware that scaling in object mode only affects the object, not the geometry inside the object. Before you export anything, you have to apply any modifications in scale and rotation by pressing CTRL + A and then selecting Rotation & Scale. If you export the level without doing so, the modifications will not be applied and proportions will be wrong in your exported level.
You can also specify by how much you want to scale/rotate/translate. After initiating scale and selecting your axis, type a number (blender units or degrees, depending on what you're doing.) A 90 deg. rotation around the Z axis would be S -> Z -> 90. Moving your object up by 3 blender units (BU) would be G -> Z -> 3.
Now you should know how to create objects and move them around. These objects will contain the geometry of your track. You could create one object for the base geometry and then many small objects for decoration and things that will be represented more than once.
These basics will also help you in the next step in which we will take a look at the Edit Mode. This mode allows you to change the geometry of objects.