Always support your analysis with "evidence" from the text:

Clearly Lismahago is meant to be a representative of Smollett's views.

Avoid simply retelling the plot with no analysis of your own:

Before their arrival in Scotland, the party meets up with a Lieutenant Lismahago who proceeds to fall off his horse before telling a fantastic tale of adventure in the wilds of North America with some native "savages."

Always use the text as evidence to support your claims and ideas:

Lydia's perspective on the places she visits privileges the beauty of nature and the landscape that surrounds her.  For example, she describes the Downs as a "charming and romantic place.  The air is so pure; the Downs are so agreeable; the furze in full blossom; the ground enamelled with daisies, and primroses, and cowslips; all the trees bursting into leaves . . . the mountains covered with flocks of sheep . . ." (27).  Moreover, Lydia's descriptions can be contrasted with those of Bramble, who seems fixated on public hygiene and economic efficiency.

Smollett characterizes Lismahago as a fool and  petty  scoundrel, as evidenced by his refusal to react to anti-Scots writing.  To react, Lismahago says, is "shallow policy--it would only serve to make the satire more cutting and severe; and, I think, it is much better to let it stand in the window, than have it presented in the reckoning" (198).

Paraphrasing is fine:

Bramble's political attitudes reflect Smollett's.  Both men's attitudes are those of a conservative, pro-union gentry class.

Always state your thesis as clearly and succinctly as possible right at the beginning of your response:

The Scottish character Obidiah Lismahgo displays his "Scottishness" with a sly and modest rhetoric.  Lismahago's adventurous stories and experience as a lieutenant impress and win over the eager ears of the Bramble family.  Although Lismahago favors and represents the Scottish nation and people, the manner in which he displays his opinion is full of facts, educated and well-thought-out, and not threatening toward the English.

Avoid "I believe" "I think" "It seems to me" phrases. In English lit. (and other) writing, it's assumed that all the ideas expressed are "your own."

Use present tense when discussing the text: "Bramble's political attitudes reflect Smollett's."
not "Bramble's political attitudes reflected Smollett's."

Proofread carefully. Don't just relay on spellchecker.