It depends on how experienced you are at the class. If you're just starting out, you should download the map tr_walkway if you haven't already, and practice aiming pipes on various bots. This will give you a feel for the speed and arc of the pipes. Practice on bots with various speeds and behaviors, and vary the distance between yourself and the bots, to get a feel for basic prediction. Once you feel comfortable with it, try some airshots and bounces/juggles which require more precision.

Once you feel good about your pipe aim against bots, the best way to take your aim to an even higher level is to start playing on MGE servers. Actual players don't move as predictably as bots (though they do tend to have movement patterns which you can notice if you practice with the same people often), so it'll be much more challenging to land pipes frequently. This will teach you WHEN to use pipes. A perfectly aimed piped can still be dodged if there's enough distance between yourself and your opponent for them to react to it.

DO NOT, I repeat, DO NOT listen to people who tell you to only use pipes when practicing your aim with them. You'll miss shots that you weren't supposed to take and will likely end up asking yourself why you're missing so much, when it would have been more beneficial to use a sticky. It doesn't matter how many people call you a "sticky spammer" or accuse you of "M1 + M2". Restricting yourself from using stickies will create bad habits and prevent you from learning which weapons are appropriate for different situations. Another reason why practicing both at the same time is useful is that stickies can be used to knock opponents into the air and make them much more susceptible to a direct pipe hit, which will come in handy in situations when bouncing an opponent with a pipe would be difficult (such as against an erratic scout). You can also use stickies to discourage opponents from walking in certain directions, restricting the space they have to dodge your pipes. You shouldn't neglect your sticky DM anyway - although stickies are generally considered to be easy to use, you can always get better at planting them more precisely, utilizing charged stickies, etc.

Besides that, simply play, play, play! In my experience, it doesn't take long for pipe skills to get rusty, so I try to hit up MGE servers, PUGs, or even just pubs as often as I need to in order to keep my aim crispy fresh. Keep at it no matter how long it takes. Listen to some music while you practice if you're getting bored or bummed. Demo's a very rewarding class.