Summoning a spiritual dragon would be dependant on your practices. One way you can summon one is to, if you can make barriers/shields (which ever you wish to call them, then try and do the same technique bit reach that in a stream to either form your own temperary spiritual dragon to protect you, or to call out to one.
Dragons are astral entities. They arent physical beings. They can't typically be summoned, unless you've already been working with them. Even then, you would call them. They only respond if they are interested. Dragons should be considered your equals and given respect if you expect to be treated the same way. The may aid in some of your rituals and spell casting, but they also require energy to sustain them.
why do you need to summon one? you could [and should] just astral project and talk to them yourself. if you need one for a spell, just call for the dragon to help you instead of a deity. if you need them to charge an item, leave it on your altar overnight in case they come by to enchant and object.
summoning usually pisses them off, so be sure you know which dragon you need, and have a good reason. if you want to summon one to see it, or because you want your ex back, they'll most likely attack you.