dual boot – how to install ubuntu on second drive when pc isonly bios legacy

1-My pc support only bios legacy.
2-On the first drive I run windows 10 .
My questions are:
-How to install ubuntu (I downloaded ubuntu on a usb)?
-Is after installing ububtu I need to download grub bootloader separetly, or it will install automaticly ,so i could select which operation systems will be load ?