Every webcam and every imaging situation will be different, so there are no hard and fast rules. What follows are some suggestions for those just starting out based on my experiences with the Philips SPC900NC webcam and the Celestron NexStar 6SE and the Celestron 9.25 SCT AVX.
First start up you computer with webcam attached, open your capture software like WxAstrocapture and select your camera. The next and most important task is to actually find the object you are searching for. Then you have to get it as accurately centred as possible and make sure that your scope is tracking it.
To centre an object like Jupiter my routine has been to put in my 32mm eyepiece, centre as best I can, then take it out and put in a 2x Barlow and the same eyepiece, centre it again then replace the eypiece with the 10mm eyepiece and centre that. The 10mm eyepiece is pretty rubbishy so the view is very fuzzy, but that doesn't matter as long it is centred. I then take the eypiece out - leaving the Barlow in - and replace with the webcam. With luck I should see something on the computer screen. If I don't then I usually take the 10mm out and put in the 32mm - no need to take out the Barlow - recentre, replace with the 10mm, recentre then put in the webcam.
However, I have found a much better approach is to use a reticule eyepiece instead of the 10mm. This is an eyepiece with a single or double x etched into it and, if you go through the procedure above, but use this eyepieces instead of the 10mm and focus and place your object at the centre of the cross you will then have a very good chance of seeing it on your computer screen when you then put in the webcam. I have a Skywatcher 12.5mm illuminated reticule eyepiece and this works very well, despite having only a 40 deg apparent field of view. It can't be used for anything other than this centering exercise but is well worth it.
So on the screen I should see an out of focus Jupiter. Gentle tweaks to improve the focus and, if it isn't in the centre of the screen, and it seldom is, I then adjust the rate speed on my direction buttons before trying to centre it. On my Celestron NexStar 6SE and the 9.25 these rates go from a very slow 1 to a far too fast 9 - well far too fast for webcam adjustments. Usually 3 or 4 or 5 work quite well though because of backlash in the gears one direction arrow is very slow in reacting and I have to be careful not to overshoot. Also, with the Alt-Az mount that I had it was seldom possible to centre properly without having it drifting - the wedge improved this situation considerably. This drift will depend on what part of the sky the object is in and how the mount 'feels'. But having it completely centred isn't that important provided that it is reasonably steady and well focused.
Different objects require different settings and sometimes even when you have been very careful and are certain that you had it centred in your high-power eyepiece you may not be able to see anything. If this happens then the first thing I try is to select the Properties, Exposure and turn up the brightness. If that doesn't work then it may be that the shutter speed and/or the frames per sec (fps) are not set correct. For both Jupiter and Saturn with the Philips SPC900NC I have found the best seeting to be 1/25 sec for the shutter speed at 10fps whilst for the Moon, which will usually be very bright, 10fps and 1/100 sec for the shutter speed with the brightness turned right down - a Moon filter may need to be attached to the webcam.
The Brightness will depend very much on the seeing conditions and it is always best to have it a little dim rather than too bright. Similarly with the Gain, choose a value which isn't too bright, possible less than 40. I have had best results with the Gamma low or even zero, otherwise there is a danger of the resultant image having ringing. With the colour option try 100% for the Contrast and 75% for Saturation. For both do make sure the Colour and Freeze boxes are ticked. And don't forget to untick the Colour box for the Moon, unless you want to introduce artificial colour.
Because Jupiter rotates so fast you should limit your video to about 150 sec. At 10 fps this means that you will end up with 150 frames. Saturn also rotates quite fast but with fewer definite features on the planet itself you can afford to take more frames in order to get better definition on the rings.
With the Moon the exposure length may be determined by the tracking ability of the scope and the steadiness of the atmosphere, but you generally won't need more than a few hundred frames. The brightness level can vary considerably depending on the phase of the Moon and you will almost certainly need a Moon filter for the full Moon. Because the Moon is 'large' you will only be able to capture a small section. You can get a complete photo of the Moon if you carefully select all the sections, making sure that they overlap a little, process them as usual in Registax, then stitch them together in MicroSoft ICE. This is free software originally developed for combining landscape photos, but it works really well for astronomical objects.
Finally before capturing your object check the focus and give it time to settle properly between each tweak. Good results will depend on the viewing conditions - steadiness and clarity of the atmosphere - and the level of light pollution which also includes the Moon, and how well your object is focused. For more information and comments see the blog.