If you want to write epic fantasy, you must read epic fantasy. Experts in the industry are consistent with their advice that you must be an avid reader of a genre to write well in it. The books listed below will teach you about the genre and help you get a feel for all the different things you can do with it. 

There are only a few books noted here, because most likely your portal fantasy will owe more to its genre combination than this genre. It is especially important to look at what is hitting the top of the list when you type in portal fantasy. If your book doesn't fit with these (currently mostly romance, erotica, and LitRPG), then I wouldn't bother using portal fantasy in your advertising/search term strategies

For this exercise, you should have a document open where you can write. Now copy the text in the box and paste it into that document. This will be the blueprint for your story.

Portal fantasies are a great way to take your reader away from their world for a while. If that's what you read for, then it might be a good fit for you. And it works as a mechanism for nearly every other genre, as well.

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...