First, you need to locate where exactly that package is, which you seem to have already done (well, by trying to install the standard bundles). Anyway, this is how you need to formally do it. Running
$ yum search todonotes
at some shell prompt gives:
Warning: No matches found for: todonotes
Looks like it is not part of any pre-packaged bundle, so you need to install it manually. You need to find this in CTAN.
Go to the corresponding CTAN page, https://www.ctan.org/pkg/todonotes. The .zip file download link is at bottom left. Download this file in some local directory. say, in the Downloads directory in your home.
Go to the corresponding directory, unzip the .zip file (unzip todonotes.zip). Move inside the directory todonotes (cd todonotes).
As you can find (ls -l) there is no .sty file there, there is only an .ins file. In order to generate the .sty file, run the following command:
$ latex todonotes.ins
This generates
todonotes.sty.
Now, we need to put this at some appropriate place.
In order to find the suggested location, run:
$ kpsewhich -var-value=TEXMFHOME
This is likely to generate something like, /home/username/texmf.
Following the TeX directory structure, you should place your file in a subdirectory like ~/texmf/tex/latex/. This has the advantage that it is not necessary to update the package database as TeX searches your personal texmftree directly. See this post for details.
So. create a directory ~/texmf/tex/latex/ (mkdir -p ~/texmf/tex/latex/) and then copy the complete todonotes directory under it
(cp -vr ~/Downloads/todonotes ~/texmf/tex/latex/).
You can then verify which file will be used with:
kpsewhich todonotes.sty
The above is should show something like:
/home/username/texmf/tex/latex/todonotes/todonotes.sty
Now, it you try to use the todonotes.sty file inside any LaTeX file, no problem should occur.
Best Answer
The package
undertildeis marked as having an unclear license. As such, it can't be included in TeX Live (which only includes free software). You will have to download the package from CTAN and install it manually: see How do I install an individual package on a Linux system? for more on this (look in particular at installing a package manually).