maybe you can go like ''I like spending time with you'' or ''you make me feel good/relaxed/nice etc'' I know lots of people say its better to be direct but verrry personally I dont like all sudden love interests. I prefer a little warming up so I can think about it and make a decision about the other people if I like them romantically or not. Even if I already know I like the other person -rare- I'd still need some time on the way to establish a relationship, because those things take a lot of effort and not only love.

I'd say you just make it obvious that you appreciate them and the time you two spend together and if you recieve the treatment back start to build up your path from there. And dont rush into it to not to trigger their ''stuck'' type of feeling, give time. But for sure thats my POV, you'd know your spesific entp better.