The following is, in general, the basic format, wording and layout acceptance and regret for a party, dinner, or wedding:
Mr. and Mrs. James Smith
accept with pleasure
the kind invitation of
Mr. and Mrs. John Carpenter
for Saturday, the fourth of July
at seven o'clock
-or-
Mr. and Mrs. James Smith
regret they are unable to accept
Mr. and Mrs. John Carpenter's
kind invitation for
Saturday, the fourth of July
at seven o'clock
accept with pleasure
the kind invitation of
Mr. and Mrs. John Carpenter
for Saturday, the fourth of July
at seven o'clock
-or-
Mr. and Mrs. James Smith
regret they are unable to accept
Mr. and Mrs. John Carpenter's
kind invitation for
Saturday, the fourth of July
at seven o'clock
So, there we are, just the basics. Variations of this format may be looked up in any complete book of etiquette. Be sure to use quality note paper (can be plain ecru which is not expensive) and use a fine tip, black ink pen. Best wishes and happy writing:))