Knowing how to help children who are stuck on a word can be challenging. It is easy to revert to simply urging them to just "sound it out". However, we want to help children develop the skills that lead to independence and "sound it out" is not quite explicit enough.
Help your child blend the sounds from left to right. Say each sound slowly. Have your child blend the sounds together independently.
For example, when your child comes to the word "sat" and looks to you, you could put your finger under each letter and have them slowly say the sound each letter represents while moving your finger under each letter, "sssssaaaat". Then quickly move your finger under the letters from left to right and have your child blend the sounds together.
If you come across a word with a phonics concept that hasn't been taught yet, you can simply give your child the sound.
For example, if you are helping your child read the word "bread" and they don't yet know that "ea" can represent short e, simply say:
"In this word the "ea" represents the sound /e/"
"/b/ /r/ /e/ /d/ ... what's the word?"
If your child comes to a phonics concept they have already learned, but they can't remember in the moment, I recommend you simply give them the sound to help them through their reading but make a note to practice it later.
For example, if they are reading the word "stay" and try sounding it out as "stah-yu", I would simply remind the child that "ay" represents the long a sound. Later on, I would review "ay" and practice some words with this phoneme.
The next time I go to read a book with "ay", I would remind the child ahead of time that there will be "ay" in the book.
"Do you remember what we practiced with a, y?"
""ay" represents the sound 'A""
"Let's practice these words with "ay" so you are ready when you come to it in the book again."