How to Remove the Instructions Text in word in VSTO

Try to replace doc1.Content.Find.Text = ""; with doc1.Content.Find.Text = "*";

Try using a Find object based on the Selection object :

Microsoft.Office.Interop.Word.Selection wordSelection = app.ActiveWindow.Selection;
wordSelection.HomeKey(WdUnits.wdStory);
Find fnd = wordSelection.Find;
fnd.ClearFormatting();
fnd.Replacement.ClearFormatting();
fnd.Forward = true;
fnd.Wrap = Microsoft.Office.Interop.Word.WdFindWrap.wdFindContinue;
fnd.Text = "";
fnd.Replacement.Text = "";
fnd.set_Style("Instructions");
fnd.Execute(Replace: WdReplace.wdReplaceAll);