1. Knowledge of the game
A good IGL should have a really deep knowledge of the game (timings, money, expected income/outcome after crucial rounds), and know what type of style he would like to use, and of course he should find players that will adapt to the playstyle.
2. Picking the right players for your ideas and their roles
To be a good IGL, you have to know your players in every way, you need to know how to calm them down or motivate them. You should work on their weaknesses and know their roles and how they can act after your call. Also you have to have good manners. You should give advice on improving their current individual skills, plan practice sessions for each of them. As the leader of the team, you need to make sure the players completely understand your strategies and make as few mistakes as possible. You have to be the guiding hand. And remember to make sure your players know their roles and what they should be doing.
Let your players trust you and trust them yourself. Always be open for any ideas from your players, if they know how the other team plays, listen to them, but you are the one putting everything together to make a good call.
3. Will to work hard and help team grow
A good IGL should be motivated to work hard and to improve overall skills of the team, for example researching and watching demos of the next team they are playing, from that you can have some anti-strat rounds that you can use in any situation.
Having a dry run for new strats on an empty server with your team is rather helpful too. Most of the time when you're trying something new, don't think about winning the scrim but rather how the round was played, if everything was ok or if there was something to work on. If you keep loosing, go back and have a look for the mistake.
4. In-Game Calls
Your calls are important for the entire match. Decisions you make during the freeztime or tactical pause should be fairly quick, and your team should always know what they are doing next. If you die during a round (usually your job is not to make as much kills as you can!) don't watch how your teammates die/kill, but rather focus on the next round, calculate the money and think about the next call you are going to make! What style your enemy is playing and how to counter it.
ALWAYS REMEMBER YOUR CALLS ARE THE MOST IMPORTANT THING BEFORE A ROUND STARTS!