For the portal fantasy blueprint, I'm going to use the Three Act Story Structure with a few touches of Campbell's The Adventure of the Hero thrown in for fun.

The first question for historical fantasists is "What history shall I add fantasy to?" Here's some sites to help you pick.

Khan Academy's World History Course — Khan Academy is a non-profit organization aiming to provide free, quality education to everyone

Do you spend all day thinking about other times, perhaps when magic still worked? When faced with a historical mystery, do your thoughts immediately turn to magical answers? Do you think Pride and Prejudice is a peerless masterpiece, yet could still be improved? Maybe by the addition of zombies?

If you want to write in a genre, you have to read that genre. Experts in the industry are consistent with their advice that you must be an avid reader of a genre to write well in it. In addition to the suggested fiction, I've included a number of books about the military itself from different eras, as that is the kind of knowledge one needs to be successful in this genre.

When you think of history, do you think of battles won and lost? Did you think Gandalf's charge with a thousand Rohirrim heavy horse ill-advised since they faced massed infantry with set spears? Did you love Game of Thrones but choked on your drink when they put the trebuchet in front of their defensive positions?

Most genre names aren't hard to interpret, and military fantasy is an especially obvious example. In military fantasy, the military takes center stage. Your protagonist will be a soldier and they will do soldierly things. Fights, battles, melees...

This blueprint utilizes the Hero's Journey story structure, a structure created by Christopher Vogler that is uniquely suited to fantasy. The terms that have been metaphorized to apply to modern fiction can be applied literally in fantasy. Look to the original structure Vogler...

Fantasy is such a broad genre that this question is much better answered in the subgenre you want to work in. But for the people who are just poking their toe into the deep waters of fantastic literature for the first time, I'll do my best.

My first few novels were all intrusion/urban fantasy. I think it's a great genre for beginning novelists, as you're given half the setting for free and can dive right in without all that pesky worldbuilding.