I remember the first few times injecting my daughter in public and feeling awkward about it. I guess it's like anything that you do for the first time that may cause people to "look", it feels difficult and you're unsure of yourself but after you've done it a few times it's not so difficult after that. I guess if you're feeling apprehensive it might help to have a good friend with you who knows you take injections, that way you won't feel on your own the first few times and have some moral support. Try to be discreet but at the same time please don't disappear into toilets to do the injection.
If the people are with you, say for example you're eating out or something like that and they're at the same table, then I'd politely say "excuse me whilst I take my insulin" and then go ahead with as much confidence as you have to quickly and discreetly take the injection. It won't be long and most people you know will understand what taking your insulin involves. My experience is that most people you know will be interested and ask questions, be prepared to give some explanations and to talk about your diabetes. I think people are genuinely interested and talking about it probably makes them feel more comfortable as they're the ones sitting there not doing anything whilst you're busy doing your injection.
People you don't know who aren't directly with you, for example other diners in a restaurant, barely notice and if they do I've never had anyone say anything to me in the six years my daughter has been taking injections. I have heard other diabetics having people comment or complain so be prepared with an answer if this was to happen but I think it is rare for that to happen. Remember you have as much right as anyone else to take your medication with your food, the person complaining is the one with a problem, if they don't like it they shouldn't have been looking at you in the first instance. They can change their line of sight if they don't like needles, you can't change the fact that you need insulin.