In general the interdental brushing technique involves the following:
Place the tip of your interdental brush at the point where you wish to insert it between your teeth and probe it gently to find the best angle to begin to insert without needing to force it. You will find some angles work better than others.
Before you insert the brush tip too far first rotate it to a more horizontal angle, this will allow for better cleaning and more control.
Slowly and gently push the brush into the interproximal area between your teeth until the other side is reached. If you feel resistance or pain it is best to stop and carefully try again. Use a back and forth brushing motion to clean your teeth and gums, usually once will be enough to dislodge any matter that may have accumulated. Be sure to rinse the brush thoroughly afterwards, and replace the cap if it came with one.
If you find inserting the brush between your teeth is impossible, you may need to select a smaller size interdental brush. Interproximal spaces can vary greatly between people, and even between teeth in the same mouth, so tapered brushes or variety packs might be a better option if you are having difficulties. The most important thing to remember is that when inserting the brush between your teeth you do not force it into the space but insert it gently to avoid damaging your gums. Although some bleeding may occur initially, this will reduce over time as your gums become healthier. If you're unsure or if you continue to have difficulty then your dentist, dental hygienist or dental therapist can offer you more advice and instruct you on how to use interdental brushes in your specific case.