I am using SketchUp (Make 2018) for the first time in my life and everything looks quite easy to use.
However I am struggling since 3 days to understand how to cut a woodwork component (a frame part) to 45 degrees at its edges… 
Maybe I am creating components in a wrong way?
I model the single frame part by designing its lines, grouping them all together and then transforming the group into a component.
Is that correct? 
The easiest way to model a frame is by drawing the overall shape of the frame, then the frame section profile perpendicular to that, and then to use the FollowMe tool. You can then get the dimensions of the needed parts out of the finished shape. This is especially useful if he frame isn’t rectangular.
It would help if you gave us your model or at least a screenshot to see what you are having trouble with.
As Box wrote, seeing your model file or at least a screenshot will help us help you. The exact steps you need to take depend on the shape of the frame part you are trying to draw.
There is no SketchUp 2018 Make. Which version are you actually using? SketchUp 2018 Pro, and eralier Make version, or the web-based version?
If you are going to make a component anyway, it’s a waste of time to first make a group of the selected geometry. Skip making a group and just make it a component from the start.
@Box @DaveR you both are right sorry. 
First of all I checked again and we are using here SketchUp Pro 2018.
Here is the component and I need to cut it 45 degrees at the right edge (so that four parts of it can create a frame).
How about sharing the SKP file. It’s difficult to tell what the profile looks like.
Best guess from your screen shot is that you’ll need to draw a face at the miter angle and intersect it with the frame member.
By the way, as appealing is it might be to apply textures to your model as you go, it’s better if you hold off on that until you have the modeling done. The textures can make it difficult to see what you are doing.
OK, here you go.
Cr4z33 Mirror.skp (77.9 KB)
Thank you. One moment.
Which version of SketchUp are you really using? SketchUp 2018 Pro?
OK. So you can edit your profile so it doesn’t indicate Make.
Yeah did that already thanks.
Doesn’t it show up?
Oh by the way forgot to say that also left edge needs to be cutted 45 degrees.
First, you should correct the blue reversed faces.
You can use the method I described above to miter the frame. Draw a rectangle and put it at 45° to the end. Use Intersect Faces and then erase the waste.
I drew the rectangle and rotated it while it was outside the component.
Then I selected it, used Edit>Cut, opened the component for editing and used Edit>Paste in place.
Select all the geometry, right click and choose Intersect Faces>With Selection.
Erase the waste.
Thanks to both gonna try immediately. 
Sorry how should I correct that? 
I can’t figure out what blue reversed faces you are referring to.
Change the face style to Monochrome (View>Face Style) so you can look at the faces without the texture applied. This is one of the reasons why I told you before not to apply textures right away.
Then open the component for editing (right click on it and choose Edit Component or double click on it with the Select tool.
Right click on a white face and choose Orient Faces.
OK done thanks.
So I started following first that animated GIF, but I ended up again differently.
Edit
Pic doesn’t show properly what I see actually in Sketchup.
The WHOLE component has blue lines when selecting just the external part.
Is that the orientation you want for the miter? Assuming you have the cutting plane inside the component, it looks like all you have left to do is erase the waste and the rest of the cutting plane.
If you only select the component but don’t open it for editing, it will show the blue lines. You need to open the component’s “container” to get at the geometry inside.
Component selected:
component opened for editing:
What Box showed was UN-grouped or a object that was not a component. The blue lines are a border box informing you that the object needs to be double clicked in oder for you to modify it. Try reading about using them in order to modify and edit them … Working with Components in SketchUp ~ https://help.sketchup.com/en/search/content?keys=working+with+components
