Taglines aren't always separate sentences. They’re often part of the sentence to identify the speaker to the reader. Periods are used to end sentences while commas are used to connect them. But taglines needn't just be "he said", "she said". Other words such as "shouted" or "whispered" can be used in the proper instances, and taglines can also include an action.
"I jumped this high." Derek said. Raising his hand in the air. WRONG
"I jumped this high." Derek said, raising his hand in the air. STILL WRONG
Look at where the periods are, ending the sentences, particularly in the second example. 'Derek said, raising his hand in the air' doesn't even make sense. To write this correctly:
"I jumped this high," Derek said, raising his hand in the air. RIGHT
What happens if you want your tagline to come at the beginning? Follow the same guideline.
Running into the house, Angela yelled. "I'm home!" WRONG
Running into the house, Angela yelled, "I'm home." RIGHT
The second example is correct because the words 'I'm home' are the ones Angela is yelling out. The first example suggests that Angela was yelling as she ran into the house, and that her words are part of a different action because the period ends the sentence after "yelled". If Angela did indeed yell before announcing she was home, a bit more clarification is needed. Something like the following would work:
Running into the house, Angela yelled. “I’m home,” she said, lowering her voice.
What Angela may have yelled before her announcement, or why she did it, I’m not sure, but you have the clarification.
Notice also that 'she' isn't capped. Unlike 'I', neither 'he' nor 'she' is capped. They're pronouns whereas 'I' is a noun, a name.
When using a pronoun, is it ever okay to cap it? Of course. When it comes at the beginning of a sentence:
He said, “I don’t have any rope.” RIGHT
“I don’t have any rope,” He said. WRONG.
The punctuation is correct in the second example, but capping he is like capping a Random word in the Middle of a sentence. It might be okay for Facebook memes, but it’s not okay for your polished manuscript.
When using a question mark, make sure it goes after the question, don’t separate the two.
“Where are you” I asked? WRONG
“Where are you,” I asked? WRONG
“Where are you?” I asked. RIGHT
In this case, the question and the tagline are indeed two separate things because ‘Where are you I asked’ isn’t the question. Even if you’re repeating yourself, if someone didn’t hear you the first time, the question is no longer a question, but a statement:
“I didn’t hear you. Say that again.” The static on Jonna’s phone crackled in my ear.
“I asked where are you.”
Most likely though, if you were to repeat yourself you would say something like, “I asked where you were” or you’d simply ask again. “Where are you?” To add to the confusion, questions don’t always need question marks. It depends on what you’re doing. Note the following:
“Where is he?” I asked. RIGHT (speech)
Where was he, I wondered. RIGHT (statement about wondering where he is)
Where was he? I wondered. WRONG (two separate things)
Where was he I wondered? WRONG (this is not the question since you know you’re wondering)
Punctuation and where to put it can be confusing, as well as which words to capitalize, especially when writing dialog. With careful attention and practice, it’ll get easier, and in no time you’ll be a pro.