I saw your request for a review on the newsfeed and thought I could offer some assistance.
I have some experience playing card games - canasta, samba, president, etc. It is from that background that I approach my review of your game instructions.
Overall I think your instructions are clear enough to follow. I think they could be improved by use of Writing ML (by bolding and underlining headings and subheadings to make the text easier to absorb) and I have a few other minor suggestions.
The first sentences confused me. I know they are designed to introduce the game, where it originated and to categorise it within a family of card games, and it might just be me, but it felt like it could be clearer.
PlayLifing is a playing card game by me, Julius the Jules, based on life decking in combat-based trading card games. The deck takes all damage, but since this is played with a poker deck, there is a shared Life Deck(in PlayLifing, it's called Play Life, or the Life of the Game).
I think if you're introducing the game in a set of instructions like this, especially when the instructions could then be transferred from one medium to another (e.g. taken away from Writing.com and posted on the internet or printed in a book of card games) you'd refer to yourself in third person.
Also, while the game may have been inspired by combat-based trading card games (I assume the likes of Pokemon would be classed as such), it doesn't use trading cards and so this seems irrelevant.
A poker deck is a standard deck of 52 cards. I'm not sure why it requires specification as a poker deck.
Therefore, incorporating some of the other information you provide further down, I would rephrase this as 'PlayLifing is a card game for two players, invented by Julius the Jules, that utilises life decking and is played with a standard deck of 52 cards. Players alternate between attacking and blocking, and a shared life deck takes all the damage.'
The objective should not be under the rules heading. Well, that's my opinion. The objective is the point of the game, and the rules are how you play the game. I would bold the objective heading and move that under your introduction. I would also rephrase the 'rules' heading to be 'Instructions'. By bolding that, you can leave it as a simple heading.
I wouldn't use separate headings for 'Matches', 'Dealing', 'Setup for battle', etc. I would include all of these under 'Instructions'.
Deal 4 cards to each players' hands
This is somewhat confusing. Generally a hand in a game of cards is the total cards held by one player. However, this sounds like each player is dealt four cards for their left hand and four cards for their right hand. I would suggest rephrasing it as 'Deal 4 cards to each player' which is simpler.
I would note after the dealing that the players alternate between attacking and blocking, and then go into the explanation of how the cards are placed on the table and how the play is scored.
Once you have finished the explanation of how the play is scored, only then would I explain what happens if a player runs out of cards in their hand or the deck runs out of cards. And lastly, explain how the game is won. After this, you could note that a 'match' is typically played until one player has won two games. As a thought, how long would a game typically last? It's not necessary to note that in the instructions, but it seemed like a game could be quite short, and therefore a match won quite quickly. It's not right or wrong, just my impression.
I notice that when you use brackets, you tend to put the opening bracket right up against the preceding word, without leaving a space. For example: Attacking card is placed vertical(portrait or up and down). In that example, there should be a space between vertical and the opening bracket. This error is repeated throughout the item.
I can definitely picture people playing this game, and I think that it would be possible to follow your instructions as is, but I hope that my suggestions help to clarify and polish them for you.
This is the first time I've seen instructions shared like this on Writing.com, so I congratulate you on thinking outside the box.