Hopefully I can help you Having done my D300 and D800 quite a few times
First of all sensor cleaning takes time and care and also a clean work area away from dust.
DON'T USE COTTON BUDS
Cleaning gear
Some photographers recommend using a "Rocket Blower " but this can stir up more dust within the camera body. I prefer to use what is called an " AcrticButterfly" which is a very fine brush that has batteries to make it spin first to build up ati static before wiping across the sensor. There are also swabs and fluid one can use for really hard dust bunnies on the sensor. Another great tool is a loupe which rests where the lens goes so you can see the sensor better.
Looking at sensor thru Loupe
My stuff I use
How to
My first sugestion would be to try the cameras own sensor clean which can be found in the camera menu. Failing that you should make sure the camera battery is fully charged to lock up the mirror also in the menu.
Ok now we have the camera without the lens attached ,the mirror locked up and a clean work area.
If using the Rocket blower hold the camera face down and use Blower gently and not right inside camera.
Using the Artic butterfy brush let it spin for a few seconds then switch off and gently brush across sensor on way at a time and repeat until clean
If using swaps
DON.T USE COTTON BUDS best using sensor cleaning fluid is to put a small drop on the work surface and dip into just enough to get fluid on the swab or you will flood it. Then wipe across the sensor one way only and then discard, do this with several swabs.
Main thing to remember allow plenty of time, don't try and rush it , and be very gentle as the sensor is costly to replace.
The above equipment is not that cheap, but weigh it up against time without camera while being cleaned and cost of several "professional" cleans and can be used with any DSLR I found the capital outlay was worth it
Google " visible dust products"