I never professionally trained as a manger, but after starting my own business I’ve been put in that position a few times.
Here’s what I make sure to do every time I delegate a task:
1) Explain the objective behind the work. This will allow them to operate autonomously without having to ask you a bunch of questions.
2) Always give a due date and set it as early as is reasonably possible. Many people will start work as late as possible. If you give the work now and ask for it done in a month it’ll be done in a month. If you ask for it done in two days it’ll be done in two days.
3) Let the person know roughly how long you expect the work to take and tell them to talk to you if it seems like it will take longer. This will head-off any misunderstandings of scope as quickly as possible.
4) Break up large tasks into smaller ones. People are bad at estimating how long things will take. If you give someone 2 weeks of work and 2 weeks to do it, then they may think it will take them 1 week and thus be done 1 week late. If you broke that up into 4 tasks their estimates would be more accurate and they would see their error sooner.