The Shire Hall will not have chairs as comfortable as the chair shown BUT it is an ideal to try to get in place at home. Most of your meditation will be done by yourself at home. A chair with arms is ideal to get fully relaxed in. Spend some time fidgeting and getting yourself feeling like you will be able to maintain the position you are in for 20-30 minutes. So, it is a good plan to shuffle and tune the way you sit right from the very beginning.
Next is concentrating on your breath. My preferred method is to breathe through my nose and notice that when you breathe in, the tip of your nose feels cold, and when you breathe out, it feels decidedly warm. Try three seconds in, three seconds hold, and three seconds out. Nothing is prescriptive; if you prefer two seconds or four seconds, then do that instead.
Just mentally scan your body. Do you feel tension in any muscle? Typically, I have tightness in my shoulders and rigidity in my arms. If you concentrate on trying to relax any areas of tension, then do that next. A good way is to continue with your focused breathing and tense and relax the muscle area concerned. As in physiotherapy, if you need to use your hands to tense muscles, then do that. Don’t rush any of this. Preparation is the key to success.