The way these exercise sentences have been set is confusing.

First make sure you know what the imperative is about. It is used to express an order or a wish. And then you must learn how to form it. For both these points you might find these websites useful : On Brainscape and on Tex’s French Grammar.

Then we will try and see why you are getting confused.

Je veux manger ces biscuits.

The first reaction is to consider that vouloir (je veux) is the verb you have to turn into the imperative, in which case the imperative is veuillez
¹. So the sentence would be:

Veuillez manger ces biscuits!
Veuillez les manger ²!

But it might very well be that your exercise book/your teacher wanted you to use manger in the imperative (It depends how the exercise has been worded or what the teacher’s told you), then the imperative would be:

Mangez ces biscuits!
Mangez-les! ³

Je suis très fatigué.

Whether in French or in English we do not state the subject personal pronoun of the verb in the imperative. If the exercise was worded with “conjugate with tu” it is really misleading for a learner. It should say “use verb in 2nd person singular/plural, 1st person plural” and not name an actual personal pronoun. The sentence you want is:

Sois très fatigué.⁴

Je veux aller à Tim Horton’s avec vous!

I will make the same remark as in sentence 1 about the use of vouloir. But here I am sure you should not use vouloir because you are asked to use the first person plural and the speaker is included in this person.⁵ So in the imperative you have:

Allons à Tim Horton’s!
Allons-y!

Compared to what you have written :

• no je (no personal pronoun)
• y is placed after the verb and there is a hyphen between the verb and the pronoun.

¹ Use an online conjugator to check your conjugation, this one for example. In you answers to the exercise you confuse two verbs: vouloir and veiller.
² Note there is no hyphen between veuillez and les because les is not object of veuillez.
³ Note the hyphen between the verb in the imperative and its object personal pronoun that is placed just after it.
⁴ And you wrote je when you meant tu.
⁵ Using vouloir would be equivalent to the English “let’s want to go to Tim Horton’s with you”.