Patience my friend is the answer. It can seem very difficult but with a little practice it's fairly easy.
First study the kit and the photo-parts, are you replacing, adding on to, or simply replacing a piece that wasn't there. If replacing you will have to remove the old kit part sometimes. Don't be afraid to study this as much as possible, dry fit the parts. If your adding on to it just study the diagram and figure where it will best fit into the construction sequience.
First give your photo parts a bath of thinner to remove and oils that could interfere with the glue. Remove the part with a fresh #11 blade or a pair of snippers. Carefully file down with a jewerls file any burrs. Then use a good pair of tweesers to prepare the part. I use the ZONA set of tweesers, 25.00 for the set of 5 but trust me they are worth it.
THen you need the right glue, slow setting super glue is the stuff i use. Also i use a kicker with it so that once i have the part in place it will stay. Take a old plastic soda cap, put about 2 drops of slow setting superglue in there. Once it's in there use a tooth pick or brass rod to transfer a small amount to the part or area on the model where it's needed. Position the part and then using a brush apply some kicker so it flows down onto the glue. This doesn't disrupt the part and it keeps the brush useful.
Finally if the part is so small or is driving you absolutly crazy trying to put it on, leave it off. This is a hobby and it's supposed to be fun. As you get better you will find out new ways to do things. I have a ABER set for a Panzer 4 that i'm not touching for a long while. Good luck and if you have any questions just e-mail me. Happy BIll