When you compose a new post, blogger takes you into 'Compose' automatically. But if you look up at the right hand corner of the box, you will also see a box called, 'Edit Html', This is the box you will use to cross something out.
Pretend that you have a list of things you want to finish before the end of the month. As you complete them, you want to cross them out. Type a 'less than sign' (this is a <> sign. Now type what you want to cross out. Then type a less than sign (<), a backslash (this is a /) followed by a 'greater than sign' (>). I have tried it several times and I found that I could just type those things into my 'Compose' section and then click on the 'Edit Html 'box and back to 'Compose' and the word or words were crossed out. Actually now that I have been playing with it for awhile I have discovered that you don't even have to go to the "Edit Html' box. You can just type everything, command and all, in your 'Compose' mode and blogger will do the rest.
But I have also found that it will cross things out even if I just add a photo or post it. This is making it very difficult to show you exactly what to do. Every time I type the command it crosses everything out, but the command can only be seen in the 'Edit Html' mode. So I am going to direct you to a site I found on Blogger. I have seen them use the word 'strike', but it worked just as well with only an 's' or the word, 'del' instead of 'strike'. Try it and let me know if it worked for you.
Notice that there is a backslash at the end of the command. This tells the computer when to stop the command. If your whole post is crossed out, you may have forgotten to put in the backslash.
To underline something you have to do the same thing. Use a similar command, only change the 's' to a 'u' for underline. Don't forget the backslash at the end or everything will be underlined.
I hope this helps everyone who wants to learn how to do cross outs and underlines. Look! Even Oscar got into the action! Without those little crossed paws I probably would never have figured it out!