This most basic of groundwork techniques also wins most of the contests for experienced and inexperienced judoka alike.
For you to practice this hold, your partner lays on their back (in contest, they won't lie down for you, and you have to get them down yourself) and you sit beside them with your back against their side.
Move in as close as possible, and put the arm nearest their head around their neck, all the way, making sure that you put your elbow on the mat.
This will entail you putting your weight onto their chest, but do not worry about this.

Your partner has two arms which are a threat to you, so you need to secure the one in front by tucking it under your armpit.
Don't worry about their other arm; can't do anything about it anyway.
Okay-dokey, your legs have a job to do as well. Move whichever is lowermost up so your knee is next to your opponent's head, and move the other one out of the way so your partner cannot see them both. If they can, they may be able to sit up and push you off.
Last item, put your head right down so it is next to your partner's. DO NOT keep your head up, looking round at the sights, as your opponent can use this to their advantage. If they are flexible enough, you are at risk of getting a foot round your neck, and finding yourself being pulled back and off!!
Although both of the above illustrations show the technique, the one on the left illustrates the completed move, with your head down low and safe.

To escape from underneath (3 options)
OPTION 1
If you are underneath, you are probably flat on your back, with both shoulders on the mat.
Difficult to get much leverage from here, so the first thing is to roll onto your side, so you are facing your opponent's back. Get in as close as you can, so your stomach is against their back; throw your free arm around their waist, grasping hold of their belt, leg, your own hand, whatever. Bit of coordination required for the next bit: lift your legs high up into the air and throw them in the opposite direction - visualise windscreen wipers on a car - and at the same time pull on your free arm. The action with your legs should help you to lift your opponent and roll them over your own body to drop them on the other side.
SUMMARY - If you are being held in scarf hold on your RIGHT side, you throw your LEFT arm round their waist, lift and roll your legs to the LEFT (eg. opposite direction), do a hefty pull and basically not only roll your own body to the LEFT, but take your opponent with you!

OPTION 2

OPTION 3