When writing a 'gaara x oc' fanfiction, it's important to understand Gaara's character. He has a complex past and is initially a bit cold - hearted but has a good heart deep down. You can have the OC gradually break through his barriers. For the plot, you could include elements such as the OC helping Gaara deal with political issues in the Sand Village. Also, don't forget to add some dialogue that shows the growing relationship between them. For instance, the OC might say something that makes Gaara open up about his feelings for the first time.
First, you need to create a well - thought - out OC. Decide on their appearance, personality, and backstory. For example, if your OC is a shy and kind - hearted person, it will influence how they interact with Gaara. Then, think about the setting. Since Gaara is from the Sand Village, you can use the desert environment to your advantage. You can start with a simple encounter between Gaara and the OC, like the OC getting lost in the desert and Gaara finding them.