Asus Z 390 Secure Boot

I have an Asus z 390 motherboard and need help with the settings.
I am not above average so I would appreciate if someone could please take the time to help. The person who built this PC has done a good job for a person of my level because I am in legacy and all works. If I could learn how to use the settings I could eventually get to secure boot for more security. I don't want to make mistakes I will be sorry for like inability to boot. I can get to UEFI settings OK and have seen all is GPT. I have not changed anything till I get help. I thought over all I could get to enable secure boot. Once done, I could then learn how to return to where I am not with legacy enabled. There so many settings I just want to know to get to secure boot then back to legacy. Then I need to know how to boot off a pen drive with secure boot enabled. Right now it is easy as I am in legacy. I think I go to boot and set up priorities then place pen drive first and ssd next. First I need to know if I can get this kind of help starting with leaving legacy and getting to enable secure boot.
Thank you for listening to this unusually long post. I only wanted to explain myself. Step by step is asking for a lot but each clicik leads to another.

 

I wrote too much in the first post.
With a z 390 board, what do I do in settings to go to secure boot from legacy?