Hi Turtle_Rider,
I will try to give you some tips and pointers to improve your meditation experience.
So, first of, lets go through some pre-meditation steps, a couple of things that will ease you into the meditation. What i always find helpful is to run a hot bath if possible, and soak in it from 10 to 30 minutes, or a long, hot shower. Then, dress warmly, a hoodie would be good, and maybe a blanket for legs/feet, and if you could be close to some heat source (radiator, fire), it would be ideal. Since you ll be standing still its good to minimize heat loss, or it will be a constant nuisance to your meditation. Lastly, utilize peaceful meditative music that will provide some support for mind and emotion. I can't post links yet, so search youtube for Healing Gregorian Chants and navigate similar songs if this doesn't suit you, in general you need something calm and slow, with less emphasis on lyrics and more emphasis on the music.
Ok, now lets talk a bit about different body stances. If you are feeling tired, weary, it is best to simply lie down, body flat on the ground, palms flat on the ground, and feet close together, on a mat or carpet. This stance asks the least from you, while being highly rewarding, and it can be maintained indefinitely. If you feel less tired, or after some practice sessions, you can start using the support of a wall to lean against, legs flat on the ground, palms flat on your sides, while the upper body being vertical, yet supported by the wall. Maintaining this stance will strengthen your overall posture, but it will also exert your body, so when you start to feel you cant maintain it anymore, end the session. If you are feeling more energized, or after substantial practice, you can free the back from the wall, keep the spine straight without support (hard for long periods) and sit cross-legged, or into the various lotus positions for folding the legs.. All 3 stances are valid, but use as advised, remember, the 1st stance asks very little of you and so you can maintain it for as long as you want, while the other two you should at least in the beginning try for limited amounts of time, the 2nd stance 10-15 minutes, the 3rd stance for something like 5 minutes tops.
Now that we have cleared the external parameters, as well as the body stance, we can talk about the internal workings that will help you not to wander off.
So, the first thing is, keep your eyes closed. With eyes closed, your brain is not asked to interpret its surroundings, it rather gets closer to its own body. So know that whenever you open your eyes, the meditation will be weakened, and vice versa. Secondly, the core function of meditation is to let go of the thinking process, so whenever thinking overwhelms you, i want you to tell your mind to perform a full scan of your body, find spots in the body where there is stored tension, and release it. Classic points of tension would be the jaw and teeth being clenched, or the stomach being constricted (in the upright positions), so you simply tell the jaw to open the clench, or the back to straighten up so the stomach can expand again. After a while the clenching will unconsciously return. It is your job to simply go there again and do the same thing. A constant tool to your aid against overthinking is your breath. Instead of moving through thought processes (which we all have been trained and conditioned to give priority to in order to perform well in our jobs and generally survive), move from one breath to the next and leave the thoughts to themselves, do not further them. Breathing will also strengthen the scanning and feeling of internal organs and will help with the expansion of the body and the release of the tensions. So try to keep a constant, conscious breathing throughout the meditation, starting from shallow breaths and leading to fuller, deeper breaths.
I think this is pretty much all you need to start, remember the basis of meditation is very simple, only complicated by our own minds. I'm here to answer any further questions, please feel free to ask for clarification on points you want to go over in more detail, and i hope to hear on your practice sessions and give you feedback.
I wish you all the best.