Correct hooping techniques could well be the most important aspect for producing a quality embroidery project. When I bought my first embroidery machine I had no idea how to hoop correctly. Back then there was not a lot of information on the internet. I am one of those people who is not inclined to read a manual unless I run into problems, However I have just read through the manual for my very first embroidery machine and the instructions for hooping are vague to say the least.
Correct hooping can appear quite daunting to begin with, but with practice you will become very quick at getting your item into the hoop and producing excellent embroidery.
Firstly try to judge how tight your hoop should be to accommodate your project; you will get better at this with practice. Now cut a piece of stabilizer larger than the embroidery frame, lay this over the lower hoop, now lay the item you want to embroidery over the stabilizer. In some cases you may need more than one layer of stabilizer. Ensure that the fabric is nice and straight and even but don't stretch it. Place the upper hoop over the top and push it into place. this may require a little force. Don't pull the fabric once it is in the hoop this can distort the fabric and change the placement of your embroidery design, it is also preferable to not tighten the screws once the project is in the hoop ad this cab pinch the fabric. If it is not correctly adjusted remove the project and start again.
Once the project is hooped you should be able to run your finger over the fabric without it moving or rippling, conversely if you have hooped it to tight it can 'burn' the fabric, this cannot be repaired. If you have not hooped correctly it can cause poor registration when stitching. You could end up with your design in one place and the outline somewhere else altogether.
A good digitizer will ensure that the stitching order is appropriate so that the outlines end up where they should be but if bad hooping will cause a good embroidery design to look a mess.
When embroidering onto a garment that requires the hoop and stabilizer to be placed inside the garment, like a tee-shirt or the leg of jeans, a good tip is to use some masking tape to secure the stablizer to the underside of the hoop. This will just pop off when you push the upper hoop into place but it will save you having to chase the stabilizer around inside the garment.