Konnichiwa!
Zoom the page to see the details.
1) Where did you put the subject of your sentence??? If you omit it from the start, you'll have your sentence a little far from standards. (Your subject is من). You have to say it once in the first sentence, and in the next sentences, it's more common to throw it away, unless you wanna emphasize it again.
2) The "adverb of time" always comes at the beginning of the sentence right after the subject. (It is جمعه ی گذشته)
3) You have to use the verb of "to dine" here. [to dine = to have dinner = شام خوردن]. It's not common to say it as you did, however it's understandable to me.
4) When your first sentence is completed, you have 2 options: First)Take it as an independent sentence and finish it by putting a "dot" at the end of it. Second) CONNECT it to the next sentence by putting "and (=و)" between them. Look:
[1]من جمعه ی گذشته در رستوران شام خوردم. در آنجا کدوهای تزیین شده ی جالبی را دیدم. ه
[2]من جمعه ی گذشته در رستوران شام خوردم و در آنجا کدوهای تزیین شده ی جالبی را دیدم. ه
Actually the term of "کدوی قلیایی" is new to my ear!!!(Don't link them when you write them!) We call all pumpkins simply as "کدو" and I don't know wether "gourd" has a special name in Farsi or not. What I have used to indicate them in number [1] is: "interesting decorated pumpkins". This expression is making the matter so simple and easy to understand. However if you wanna choose the hard way, I mean to say "an interesting decoration which was using gourds", you would say:
"تزیین جالبی که در آن از کدو استفاده شده بود"
Note that "در آن از کدو استفاده شده بود" is grammatically considered a sentence, and it's describing "تزیین جالب". Here, I call it an expression. I'll talk about it in the lines below.The verb of this expression is: "استفاده کردن(=to use)". If you bring it to a passive form, you'll have: "استفاده شدن (=to be used)" and next bring it to a past perfect tense, you'll have: "استفاده شده بود". (How much do you know about passive verbs?? Do u need any lessons about it??)
Past perfect verbs are used for indicating an action in the past, which has been done before another action in the past. "To use gourds in decoration" is an action which has been done before you eat dinner in the restaurant, so you need a past perfect tense to talk about the usage of gourds.
^_^NOW, now, now, it's time to put it in the sentence. You can't simply do that by just replacing it with "کدوهای تزیین شده ی جالب" in the sentence [1]. First look at the final sentence where I've rewritten, then I'll explain more: (First one is more common and also easier to say and understand)
[3]من جمعه ی گذشته در رستوران شام خوردم. در آنجا تزیین جالبی را دیدم که در آن از کدو استفاده شده بود. ه
[4]من جمعه ی گذشته در رستوران شام خوردم. در آنجا تزیین جالبی را که در آن از کدو استفاده شده بود دیدم. ه
Look at the points below and check them in the number [3] or [4].
You want to mention this: "I saw … ." You know that "to see" is a transitive verb. In other words, it needs an object. To identify it's object, you have to ask yourself: "What did I saw??". The answer is "an interesting decoration (=تزیین جالب)".
Object is always a noun which can be followed by a few adjectives after it (جالب(=intersting)). In that case, they make a "گروه مفعولی(=objective group)" which is a technical term. However I'd rather call it simply as "مفعول(=object)". So when I say object, I actually mean the "objective group" which can include more than 1 word. Here, the object is "تزیین جالب". The rest of our expression, which is " که در آن از کدو استفاده شده بود " is a clause which describes the object. If you omit it, NOTHING would happen to the sentence structure. Whenever you wanna explain more about a word, put a "که(=that, which, who, whom)" after it and describe it!!! (easy!^^)
When you diagnosed the object, you have to put a "را" immediately after it. That's an important rule!!! And this is what I want you to always remember!!! (Put "را" immediately after object of the sentence!!!) If you look back at the sentence number [1], you can see that our object is "کدوهای تزیین شده ی جالب" and "را" is immediately after it. If you had just replaced "کدوهای تزیین شده ی جالب" with "تزیین جالبی که در آن از کدو استفاده شده بود" in the sentence number [1], you would have "را" far from the object!! I mean you would have this:
من جمعه ی گذشته در رستوران شام خوردم. در آنجا تزیین جالبی که در آن از کدو استفاده شده بود را دیدم. >>> the place of "را" is wrong
I think I made it pretty clear, though make sure to ask me the unclear points.