Use a thread that matches the felt colour of your patch, use a hot iron & baking paper to protect your fabrics from scorching, position & press down on your patch, rather than rubbing over it, it should adhere to your fabric in a minute or so, some fabrics may take a bit longer, the glue will only hold for a while without sewing, so for regular wear & washing, make sure you sew it down.
My iron on patches are quite detailed, if you are new to sewing, don't worry about your sewing being seen, make a feature of it, try sewing down the veins on leaves, or a few long stitches on butterfly or bee wings, just make a pattern of your stitches that will go with the image you are sewing down.
If you have a sewing machine, I recommend free motion sewing to hold down your patches permanently.
Drop the feed dogs on your machine, & use an embroidery foot, as it lifts when the needle rises, giving you freedom to position your next stitch. Go slow, with your hands to the sides of the needle, & make sure your work has support on a table, so it doesn't drag.
Practice on scrap fabric first to get used to the feel of free motion, it is quite different to regular sewing, if you have a needle up/ needle down option on your machine this works well for keeping control in difficult areas & slows down the sewing so you can get a rhythm going when you first start.