How did you become a member of a guild in the middle ages?

The usual path to guild membership started with apprenticeship. As a child of six or seven, a person would be apprenticed to a guild master who provided education in exchange for work. The apprenticeship usually lasted until the child was a young adult and adept enough at the trade or craft to work independently.

Normally, the next step was for the person to be a journeyman. A journeyman learned more of the craft or trade from different masters. A journeyman also worked independently on his masterpiece, which was a project specifically intended to show his ability.

The final step was to have the masterpiece judged. If it was good enough, the journeyman could be admitted as a guild member.

There were many different guilds, and they were not all organized identically. Some had different grades of membership, and in some cases membership was granted to the widows of guild members so they could continue family businesses, with much of the work done by journeymen under the widow's supervision.