Use PaperUI to create your mqtt things and files for items. Note you will need to have a generic mqqt thing for each device. After you have the mqtt thing it will show you the channels and the correct syntax to use in our items file.
Here is a link that has a video. First time setup of MQTT [solved] - #32 by marsic
Here’s a post with some item file examples for mqtt. It also has files for things so make sure to look at each example so not to get confused. Need help to migrate to the mqtt binding v2 in openhab 2.4 - #14 by sargeras