I personally place the sticker in position and apply a couple of strips of low tack masking tape over the sticker and onto the suface to afix to .
( i do this with anything from a 3" sticker to a 10" sign as i have a Chinese 600mm plotter as on occassion i make signs for people )
Then apply a light spray of 1 drop of washing up liquid in 500ml of water .
Turn the sticker over ( use the masking tape as a hinge ) and peel back the backing away from the masking tape hinge to half way across the sticker .
Fold the sticker back over slowly working down from the masking tape hinge laying the vinyl down and removing air bubbles as i go with a propriatory plastic squeegy , thpough a credit card etc can be used but are not as solid as the correct tool .Then once the backing is reached , peel it off as you work down using thy squeegy .
If no masking tape is at hand just peel a little of the backing paper back and turn it over to make a buffer to keep the sticker from touching the surface till you have it positioned and gently press a little down to hold it before going the whole hog with the squeegy .
Air bubbles are removed by use of a needle point and circular movement of a finger around the bubble working inwards .
If you have corregated surfaces then warm the sticker with a hair dryer to get it soft so it will mould easier into the undulations .
A " Stippler " ( Hard bristled brush ) aids this moulding process .
The same goes for rounded edges etc , where the slight heating from a hairdryer allows for a much better adheasion over time ( no lifting along the curve line ) .
For a better quality approach buy some low tack lifting paper and cover the whole sticker with it and use the top as masking tape and proceed from there .
The extra covering of the low tack lifting tape eleviates any damage to printed stickers whilst squeegying them down .
There are many companies who sell the lifting tape but i use MDP at - http://www.mdpsupplies.co.uk/
Check out their vehicle wrap and pimp up your Bolens They dont all have to be red ! .
Their digital paper is also amazing as you can design and print your own stickers just how you want them using a std inkjet printer , i still see some i did around 6 years ago for advertising on shop doors .
Though these need a lifting paper as a barrier when applying them , Print a sheet out and cover in lifting paper then cut out with scissors or knife .Peel off the backing paper and apply , once happy remove the top covering of lifting paper .
I prefer the paper tapes for application work and the opaque plastic tapes for building up layers when prepping a sign using multiple colours as it is more easily viewable through.
Hope this helps .