I would like to start adding pitch accent to my cards. How should I grade the cards once I add pitch accent?
Don't change anything about the grading process. Put pitch accent information on the back of your cards, but don't take it into account when grading yourself.
If you force yourself to recall the target word and its pitch accent at the same time, failing to recall either one will mean you must press "Again" on the card. For example, you will have to fail a card if you remember the meaning but not the accent. Piotr Wozniak's 20 rules basically advise against that.
If you consider an item that is composed of two sub-items, you will need to make repetitions that are frequent enough to keep the more difficult item in memory. If you split the complex item into sub-items, each can be repeated at its own pace saving your time.
To learn pitch accents of individual words, make separate cards. Put the word on the front and its pitch accent on the back.
For example:
| Front | Back | 
|---|---|
| What's the accent of 出来る? | 中高 (2) | 
| What's the accent of 入る(はいる)? | 頭高 (1) | 
If a word has multiple readings, include furigana on the front.
Note that many Japanese words have multiple correct pitch accents. It's often easiser not to make cards for those, since you'll likely get them correct regardless of which accent you use.
You don't need to make pitch-accent cards for every word. Include only words that are common and that you are likely to say often. Roughly the 2,000 most common words, no more.