At first, you look at them until they notice, then you look away. The first time, they’ll probably be irritated, because for all they know, you looked at them, because their hair looks silly or whatever. Then give it some time before you look again. In some sense, you can play it like you’ve just been caught looking at them, so you don’t want to be caught again right away.

Then you repeat that a couple times and try to gauge their reaction (without directly looking at them). If they catch onto you and make an ew face, then definitely abort mission. If they smile, that’s a good sign.

Then you can try to hold the stare a little longer before looking away and again, see if they smile. Eventually, you can try keeping the eye contact for a few seconds while they smile at you and then you smile back.

At that point, it’s definitely time to talk to them. They would not keep up the smile, if they had doubts about going on a date, although it’s obviously still not a guarantee. Sometimes, they only realize that they can’t follow through when it gets serious.

But this whole spiel does communicate that you’re shy about it. If that’s not how you feel, it’s better to talk to them right away.
In general, the more time you take before doing the talk, the more serious it is, because then it’s not just a spontaneous “you’re cute, wanna go out and see if we fit”, but rather a “I’ve been observing you for the past weeks and worked out that you are my dream partner please marry me right away”. So, yeah, don’t take too long before you talk to them.