Push the corner T-post and the adjoining T-posts on each side straight and upright.
Tie a piece of string between the top of the fence on the corner post to the bottom of the fence on one of the adjoining posts. Cut the string at both ties with a pair of fence cutters or side cut pliers.
Measure the length of the string with a metal measuring tape. Cut one of the T-posts you intend to use as a bracing post to that length with a hacksaw.
Attach T-post corner braces to the top and bottom of the bracing T-post. There are several designs of these connectors. The best known brands may be "Wedge-Loc" and "Speed Brace." Attach the braces according to the instruction sheet that came with them.
Install the braces, with the bracing T-post attached, at the top of the fence on the corner post and the bottom of the fence on the adjacent post.
Repeat this procedure to install the second bracing T-post.