Mosaics are a challenge, but very rewarding. I've done a few with mixed results. The key component to mosaics is planning, planning and some more planning. It takes sometime to determine the best framing for an object and the quantity of frames required. Start with a small project i.e. no more than four frames to begin with. Below is some of the software I use when performing mosaics. I'm progressively learning what it takes to be successful in this field. To do it right takes dedication and patients.
TheSky6 for all planing of frame composition. It has an advanced mosaic feature factoring image overlap for alignment and guide star selection. You can determine how many frames are required based on your imaging train field of view and object size. Very powerful features. Mosaics are very difficult without the right tool to plan your frames.
Once you've got the plan organised. You then need to decide what your image composition will be. Conventional RGB is good for starters, but you may want to try LRGB etc. Keeping in mind the more frames, the more data acquisition will be required to deliver the final image.
I use ACP for frame acquisition. For mosaics to be successful you need to be able to have precise control over where your telescope is pointing. Sounds simple and with goto telescopes, how hard can it be right?... Wrong, with mosaics we are talking about high precision pointing. What I mentioned above about image overlap for alignment is important. If your telescope doesn't point with high precision it will not be possible to guarantee overlap is achieved. The last thing you'll want is for your frames to not align correctly due to miss framing. ACP does a few things to assist the mosaic imager. Firstly, it does plate solving. Plate solving takes ab image and matches it against a known star catalogue that has astrometry calculations to determine where the telescope is pointing. If the telescope isn't point exactly where you've told it to, then the plate solving process will remedy this by informing the telescope to move x/y coordinates. I've explained this in the most basic form, if you want to know more about plate solving, let me know. Secondly, TheSky Mosaic plan can be exported into ACP to automate the image acquisition process. It will simply acquire each frame of the mosaic as instructed via the script (plate solving as it goes to ensure each frame is precisely centered as per TheSky plan). Once you've got the data for each frame, you'd calibrate (flats,darks,bias), register (align) and combine it. So you end up with each individual frame which is noise reduced etc.
Now you're on to the fun part... image registration. Each mosaic frame needs to be precisely aligned before you load it into photoshop. I use software called Registar for this purpose. Very effective tool. Once each image is registered to each other, its a good idea to set the basic background brightness levels using DDP or pixel path. Each frame will certainly not be the same brightness so you'll need to compensate for this.
Then, bring each frame into photoshop. As they are already aligned, they can be pasted as layers and manually moved into position as required. Once you are happy the frames match - in particular the brightness between each frame is a smooth transition (so it looks seamless), the image can be flattened and image processed using your typical processing routines.
Here is one of the many resources available online to get you started - http://www.astromatt.com/Articles/MosaicsPI1.html