Astral traveling is easier than most books would suggest. Most books claim that you have to have complete control over your body and to forget it completely to have access to the astral plane.
All it takes however is a good visualisation skill. But instead of trying to develop this skill by the traditional imagine a point, line, tattwa crap, start with complex things. This may sound difficult at first, but read a good book with lots of details on scenery and you will see that scenery in your inner eye. See? It wasn't that hard.
The next step in astral traveling is to open a gate to that visualised plane and to walk through it and go into the visualised image. Arrived there start exploring this world. Notice every small detail, so the world gets ever more real.
Contrary to popular believe it is not necessary to lie still to do this. It would only hinder you, since most thoughts would be on "I may not move" and "I must lay completely motionless". This motionlessness is a great accomplishment if you can do it, but it isn't very productive for getting any results. Some mages like to forget the body and lay all emphasis on the mind. See it that way: If you ignore your body too long and only train your mind, you will certainly get sick and what good would that do to you?
At the beginning it does help to be in a resting position and in a darkened room. As you go along you can incorporate astral works in normal ritual works.
Suggestions for traveling the planes
First of all you can create a temple or working area where you are going to perform rituals on the astral plane. This temple can be designed in any way you want and can be more easily adapted for the purpose of the working than your physical working area.
You can also agree to meet other mages on the astral plane to perform rituals with them. Just be sure to make an appointment with them first.
Then of course is the possibility to travel into any symbol to discover its meaning. This for example could be done with all the runes. That way you will learn far more about them than in any book. Another possibility would be to travel into the tattwas, the signs for the five elements. The realm of Fire for example is hot, active, destructive, the colours bright. The realm of Water is wet and cold in tones of blue. The Air realm is in constant motion with silvery colours. Realm of Earth is try and cool in brown and green colours. While the realm of Ether can be all of the aforementioned in black or white colours. Naturally you can also go into self created sigla to load them from the inside. If you love paintings you can also go into them to discover what the artist originally felt about them and what he wanted to say with it. It's also a good idea to travel into the sigla of gods, demons and other entities to meet them on their own ground. There you can ask for their help in specific workings or ask them for guidance and advice.
If you're extremely good at traveling, you can also try to go into music. For this you should record the song you want to experience on the astral on PC and set it to loop. For starters, it would be best to use instrumental songs that can be set to loop more easily (there is no noticeable break then). Lie down, listen to the song and start visualising. It's best to let yourself guide by the moods that the song create in you.
Further you can also go back through time on the astral plane. I would suggest that the mage goes to the astral plane and there evokes a god of time. Kronos, Saturn, Thoth, maybe one of the Norns (Urdr for past, Skuld for future), Bran or if you prefer the fictious gods, try Tarod of Chaos or Akatosh, dragon god of time. Ask the god to take you to the time you want and guard you (and of course to bring you back when you desire).
Another method would be to create a guide, similar to servitor/egregore creation with the Statement of Intent like "to bring me to the time I want on the astral plane" or whatever. You could use it similar as those ter'angreal dream rings from the Wheel of Time books. Create a servitor in a ring or whatever you can comfortably wear while you are lying and call it when you're on the astral plane. In some myths, dragons are supposed to be able to time travel. So when on the astral plane, change into the form of a dragon and thereby assume his skills and fly backwards or forwards. If you don't want to do that you can also try to simply walk back in time. Visualise for example an hourglass where the sand flows back up to symbolise going back in time. Or better yet a full clock along with date, so you can better control when to go.
Also advisable would be that you get used to the idea to change your shape on the planes. Your body may not be suited too much to travel through certain areas, so change it accordingly. Small holes could be entered in the shape of a serpent. And as stated above, dragons have supposedly the ability to travel back in time. And of course they are able to fly. Do you really think that you could walk around in a plane that is governed by Air?
When you meet entities on the plane you should react open minded. They might help you along on the realm you are or they might also give advice for workings you're planning. But you should also be always on your guard, not all entities will respond to you positively.
To get back you could either go back to the gate you came in or you could create a new one where you are and go back that way. In either case, the gate should be closed after your return as not to invite unwelcome guests.