RE "...How exactly did you "enter it" without a second computer though...."

You would use the "monitor program" to poke hex data into memory locations, starting at a nominated RAM memory address. Needed to be done VERY CAREFULLY. one bad hex digit and your program likely crash. After entering all the program into memory, you would use a monitor program , command to get tell the CPU to jump to a certain address and start executing a program there. OLD computer magazines often had such hex listings printed in them.