Any tips for somebody who is trying to learn how to shotcall?

Never do baron.

look at the situation you are in such as opponent comp, where the jg is, and even what you can grab after a team fight. ex. Baron is a big shot call but is not necessary such as if you get a good team fight early and you could get baron you have to think what can you do with baron this early. If you determine that baron will let you get tier 2 tower safely then get it but if you have doubt that you can get drag and a tower you have to decide, but the biggest thing about shot calling is dont doubt your final decision. Dont hesitate and waste time thinking about what to do you have to get used to situations through experience and make the call right there and then.

Have 100% map awareness. As much as possible, you should know where everybody on both teams is. You should also learn to monitor the positions of your creeps. Predict what the enemy will do next. Step into their shoes. Watch pro games to understand how they make calls. Try to plan in advance and take steps to facilitate your plans. Simple example, suppose drag is coming in 1 min. Remind your team to back now. This can save your allies from being forced into a bad fight around dragon or give up dragon since they dont have items or hp or mana or whatever.

Most importantly, just call a lot of shots and make mistakes to learn when to do what.

Don't do Baron if the enemy does not have an inhibitor downed.

Warding helps make smarter decisions.

Know the "typical" play style of the enemy jungler. Like how MOST tanks have slow clears making them prone to invades, how Fighter champions (such as Udyr) clear the jungle super quick and invade and steal camps, and how most assassin junglers can't clear very well but can instagib an enemy so they will invade and try to kill the enemy jungler. There are some exceptions to these rules but knowing this will help you predict where they will most likely be and if they have an advantage where they will likely go.

If your team gets a kill somewhere pressure/force an objective. Kill the enemy top laner? Shove that lane and call the enemy jungler over to cover the lane. This will in turn allow bot lane to shove their lane forcing the enemy jungler to choose (and lose) one of their choices. Or it will call the mid laner away from their lane and you can shove back there. If you kill the enemy ADC take a dragon.

If you are support buy pink wards and sweeper trinkets and actively remove enemy vision.

Watching enemy health bars goes without saying but also watch their mana/energy bars. When it gets low they are vulnerable and are limited on what they can do if you force an engage.

"Know how to play the game well." Because that's pretty much all they are talking about when they bring up "shotcalling"

Another important tip that doesn't get brought up often enough but really should, "Know if you shouldn't be shotcalling." Because you just got half of your team killed because of your pinging that Dragon and trying to get your team to "play for objectives", for example.