1. Mention that you’re single. The first step to showing them you’re interested is making it clear you’re single. You can casually drop this fact into pretty much any conversation. Mention that you’re looking for a relationship. Mention that you’re currently available. Maybe even mention what type of person you’re looking for — as long as it describes them.
2. Compliment them in a meaningful way. You don’t want to be inappropriate, but you don’t want to give them a compliment that comes across as strictly friendly, either. If you’re complimenting them in person, you can probably get your point across with your tone. And if you’re complimenting them over social media or text, a few emojis might make your intentions clearer. Before hitting send, reread what you’ve written and ask yourself whether you would think someone was flirting with you if you received the same exact message. If not, rewrite it.
3. Break the touch barrier. Again, you don’t want to cross any boundaries and be inappropriate, but you want to make it clear you see them as more than a friend. You can start by putting a hand on their arm when you’re talking, or simply by sitting close to them. Eventually, you can move up to putting a hand on their thigh or their waist or their cheeks. Cuddle with them. Kiss them. Make it clear you want them.
4. Make eye contact with them. Even though you’re probably nervous whenever they walk into the room, it’s important to make eye contact with them. Actions can speak so much louder than words, especially in the beginning of a relationship. You can create sexual tension without even opening your mouth.
5. Initiate conversations with them. You can stare at them when you see them in person, and you can like every single photograph they post on social media, but they still might not get the hint you’re interested. You’re going to have to take a bigger step, like talking to them. The more you get to know them, the more comfortable you’ll feel around them, and the better your chances are of developing a strong relationship.
6. Surprise them with sweet gestures. Bake them dessert. Buy them food. Make them a playlist. Recommend books and bands that you think they’ll love. Once you make it clear you’ve been thinking of them and are willing to put in the effort to make them smile, your feelings should become pretty apparent.
7. Invite them out with you. Make room in your schedule to spend quality time together. Make sure they know you consider them a priority. Make sure they know you’re excited to see them. And when you hug them, see what happens when you linger. See how they react when you get close.
8. Tell them how you feel. This sounds scary, but sometimes it’s the only way to get your point across. Some people are oblivious and can’t take a hint. Others have low self-esteem and would never guess you want them. To make sure they understand how you feel about them, you’re going to have to be brave and say the words aloud. 
Source: https://thoughtcatalog.com/january-nelson/2020/10/8-ways-to-get-it-through-their-head-that-youre-interested/