First, i dont think so you need an Tenancy letter with baladiya attestation for Family visit visa. Try in Al Gharaffa Immigration office where you can apply Family visit visa with Tenancy letter without Baladiya attestation.
Anyhow to answer your question,
You have to get the Tenancy document from your Landlord or the House Incharge or from the Agent, then you have to visit Ministry of Municipality and Urban Planning (Baladiya) which is in Al Sadd to get your Tenancy document attested.
First, i dont think so you need an Tenancy letter with baladiya attestation for Family visit visa. Try in Al Gharaffa Immigration office where you can apply Family visit visa with Tenancy letter without Baladiya attestation.
Anyhow to answer your question,
You have to get the Tenancy document from your Landlord or the House Incharge or from the Agent, then you have to visit Ministry of Municipality and Urban Planning (Baladiya) which is in Al Sadd to get your Tenancy document attested.