The second sentence contains a parenthetical reference to Trust Mark Issuer validation that is somewhat out of the main context but that should be a mandatory prerequisite.
I propose to make it more explicit mandatory requirement that establishing trust in the issuer must occur before validating the Trust Mark itself.
Something along these lines:
An Entity MUST establish trust in the Trust Mark Issuer by following the procedure defined in the Section 10 prior to starting the Trust Mark validation process defined below.