Hi,
I would strongly recommend that you do the embedded tutorials in the Fusion Documentation and also some of the
Self-Paced Learning to help you to learn fusion faster and better. They can be found here:

It is also much easier for the forum to help you if you attach your file AND a screenshot of what you want to achieve
and what the problem is. You can create a file to export like this:

Time spent on the tutorials and self paced learning will not be wasted. Also check out the three RULES that are pinned to the forum for further guidance.
If all you want to do is cut the curves into the bottom of what you have then this is fairly easy. Create a sketch of
the curve you want on one of the planes and project the bell shape and centre line and bottom line onto it.

Create a three point arc the size you want the feature and close the bottom of the arc with a line.


Use the arc shape you have just created to make an extrusion cut in the bell.


Then create a circular pattern with this feature to make the curves.



Now you have a bell with wavy bottom.

What are your intentions with this model? Right now it will not do what I suspect that you may want to do with it.
There is only one component in this model - the Top component. There are also six individual bodies that are not
actually joined up to anything. They "look" like they are joined up but they are not. If you wanted to 3d print this,
for example, then it would fail.
There is also quite a lot of unnecessary things going on in the timeline. This model is using the Design in Place
method - good so far, but it is completely unnecessary to create a new plane on the face of the Wall Plate you
have started with so that you can model some more of it. You can actually start your next sketch immediately on
the face of the surface without creating the plane. Also, your Wall Plate should be a separate Component.
I am not sure what you have form modeled?
You are using simple sketches to do one or two things - excellent! However you are NOT constraining those sketches
- BAD! Constraints help define the sketch so that fusion does not have to guess what your intentions are later.
You only needed TWO fillets on the Wall Arm and it should be a separate Component. If you cannot see then use the
Eye Icons on the Browser tree to turn the visibility off and use the Viewcube to move around.
You did not need to create two bells, just a single and the Shell command would have given you what you wanted.
Overall your design is not bad.
Read the Documentation. Do the embedded tutorials. Do the Self Paced Learning. This time will NEVER be wasted.
Cheers
Andrew