I think kamui231a is mainly talking about finding targets, not engaging them.
The main way to find targets is to fly your aircraft in a manner favourable to finding things, and then to be persistant and thorough with the TGP.
Flying the aircraft favourably for finding targets:
Be some distance from the target area, in the order of 10-20nm. I personally find that 15-20nm is usually best if I have no idea where the targets are (other than say a very general area of terrain). The further you are away from the target the more area you can 'see' with the TGP, and the easier it will be to slew around that area. If you're too close, you will have to slew the TGP larger and larger distances to cover smaller and smaller areas, and your field-of-view will also become smaller and smaller. If it helps, imagine a cone extending from your TGP — at longer ranges the base of the cone is large and thus covers large areas, whereas at short ranges the base of the cone is smaller and thus covers smaller areas.
Fly high above the target area, in the order of 10-20k feet. The higher you are the more you will have a 'bird's-eye view' of the target area, and so targets will tend to be spread out over larger areas. If you fly too low, you will be looking 'through' the houses, trees, various terrains, and targets all mixed together.
Find a secure area and initate a gentle orbit using the "ALT" autopilot mode. This will fly the plane in a circle that is anchored around a fixed point on the ground. This allows you to fly 'hands-free' and focus on searching for targets. Consider throttling back so that your TGP has visibility of the target area for the longest possibly time before becoming masked. Alternatively, you could try flying towards and away from the target area and iniating "ALT/HEAD" autopilot as necessary. However, if you choose the latter method, try not to fly too close to the target area, and consider throttling back when heading towards the target area to slow your encroachment.
Alternatively, if you don't think there are many SHORAD threats around (SAMs, MANPADs, AAA etc.), you could try flying low and slow (<3000ft and say <220knots) and using your own eyes and the game's zoom function to find targets. Look for smoke, dust, muzzle flashes, and weapon tracer effects and so on. This method is risky, but it works.
Using the TGP to find targets:
Set up your "GAIN" and "LEVEL" settings using the OSBs when in the WHOT or BHOT infared TGP modes. GAIN increases and decreases the contrast of the display, which makes objects 'pop' against the terrain and become more visibile. LEVEL increases and decreases the brightness of the display, which makes it easier to see things. I typically use a GAIN setting of 6 and a LEVEL setting of 2, but play around with it and find what you prefer. Also, remember to adjust GAIN and LEVEL as necessary to meet the conditions of the particular mission you are flying, as there is no 'right' setting for all circumstances.
Find an area of terrain were you suspect targets are. Mentally section-off that area based upon some prominent landmarks. For example, think to yourself I'm going to search the area from this forest to this lake and After that, I'm going to search from the forest to the town. Once you've defined the area to search, slew the TGP left to right and front to back until you've covered the entire area. This is your search pattern (for clarity, refer to the diagram). I usually try and thoroughly cover one sectioned-off area on each 'pass' of my orbit. If you don't have a search pattern or some form of repeatable method, finding targets will simply be a matter of luck. More often then not you will not find anything, and you will spend a lot of time looking at the same areas again and again. This is a recipe for frustration and makes people think that finding targets is magic, when it really comes down to technique and persistance.
As you are flollowing the search pattern, change FOV as necessary with China Hat FWD Short (CH F S), and zoom in and out with DMS as necessary. Further, regularly change from BHOT to WHOT, because sometimes targets 'pop' more in different modes. If you get confused about where you are looking, switch to CCD mode, zoom all the way out, and change to wide FOV to get your bearings again.
It bears repeating: The most important thing when it comes to finding targets with the TGP is to have a rigorous search pattern, to be thorough with zooming and camera mode switching, and to be patient.
In short:
Fly far away from the target
Fly high
Fly slow
Estabilish a good orbit or approach
Configure your infared TGP modes
Observe landmarks
Create and follow a search pattern
Constantly change levels of zoom and constantly flip TGP camera modes from BHOT to WHOT.
If after all this you are still having trouble, don't fret: Finding targets is often hard and tedious, even with a good plan and an aircraft that is designed to loiter for long periods. This is why bossy JTACs exist. :harhar:
Finally, here is my horrible drawing: