What Jimmy said about optimizing combos and okizeme is correct and insightful. There is nothing wrong with using an already optimized combo from videos of better players with your character. I strongly recommend it. There should be a couple things you want to consider when making a combo.
1) How much damage does it do.
2) How far does it carry my opponent across the screen (more importantly to the character.)
3) What kind of okizeme does this combo give me? (A setup which will force the opponent to block (for example F-Maids with Kohaku front using a Hisui chair assist), a cross up so their input will be reversed minimizing the likely hood of a standard reversal (C-Miyako is very good for this), what can I do to limit their options on wakeup (using a strong command grab so they cannot mash or shield), or the like.
4) How much meter does the combo take. If a combo takes 200% and does less than 5000 damage most characters, it's probably not worth it unless you're in max, heat, or blood heat.
5) How practical is the starter for this combo and how many ways can I start it?
When you build combos in Melty Blood, you must first consider what moon your using.
Crescent: Understanding the Crescent moon system roots yourself in being able to understand the reverse beat mechanic of the game. Essentially, In C/H moon you can "reverse beat" in strings or combos into moves you haven't used before in said string/combo. Reverse beating prorates your damage and lowers the overall damage of the combo, but is often times worth it to make difficult links for strong moves or add a lot of moves to a combo that otherwise wouldn't be able to be linked.
To elaborate, the move chain system in melty blood works A->B->C->Special Moves. Within that, it goes standing moves -> crouching moves -> "extra moves". (Moves that are "extra moves are like 4B, 4C, or 4C, as in nonstanding-noncrouching normals), meaning that the natural chain of occurrence looks something like 5A 2A 5B 2B 5C 2C 3C Aerial combos, assuming all those moves can connect on hit (this depends on your character.)
Reverse beating allows you to "go back". This is arbitrary, but, if you do a C move but then are close enough to hit a 5B, doing 2C -> 5B would be a "reverse beat". Thus, you can then go 2C *5B* 2B 5C 3C j. BC j.BC Air Throw all at the cost of that one single reverse beat. If you couldn't reverse beat, you'd have to do something like 2C 3C j.BC j.BC Air Throw. Generally a combo with more normal hits will yield more damage, so in this situation you'd have to compare the likelihood of hitting these combos, the damages, and the other categories I listed at the start of this post to see which is more "optimal."
Half: Half moon is very unique in that each character gets a 6AA dial which acts different than A-moves in the combo chain. This is not my area of expertise, but, the 6AA is a special move that acts a B and C move in the combo chain if I recall correctly. Half moon can also reverse beat. Many characters in this moon can also whiff 2A or moves slightly in their combos to reset their move chain mid-combo (though this should also be possible in crescent moon and full moon, it is not nearly as common) Meaning, they will actually do moves that link in combo but because they whiffed 2A or walked, they can use any move again in combo because their combo chain was cancelled (even though they are still comboing their opponent). This isn't quite my specialty, so you'd best ask a half moon player more about this.
Full: The full moon combo system is completely different from Half/Crescent moon. Full moon cannot reverse beat whatsoever, meaning not only can they not go back from C to B or A moves, but they cannot go from crouching moves to standing moves, extra moves to normal moves, or special moves to normal moves. Thus full moon moves overall do more damage to compensate for the lack of more moves used in combo. Full moon mechanics follow the strict A->B->C chain akin to many other fighters today. Full moon does occasionally have loops (such as F-Wara's ground loop) which he resets his move chain by walking, but it's rare. It's the most strict move chain system, but this also makes it the easiest to use/learn/make combos for.
In all moons, when you jump it resets your move chain which is why most combos do j.(A)BC j.(A)BC Air Throw to end a combo in general. Most times people omit the A because it can be dramatically harder to hit for minimal damage. However, for a character like H-Mech, j.AAC j.AAC is much easier than j.BC. Of course, some characters have some strange special moves or charge moves that make it more advantageous to do other aerial combos or enders as per the guidelines above (usually for extra damage, corner carry, or okizeme). Some characters even have enough hitstun on their air moves to land and then re-jump in their combos. Re-jumping resets their move chain and their amount of jumps so they can even do an aerial again.
And admist all this, in this version(v1.07) and v1.05 of the game OTG relaunch exists. This means some moves carry a property to relaunch your opponent from an untech-able state. Most 2C's in the game and some special moves induce untech-able state (also called hard knockdown - meaning they hit the ground and can't tech), and you can relaunch them with special moves or otherwise to continue your combo. These combos are deadly because they almost always carry you to the corner for corner pressure and add a lot to a character.
Overall, the combo system in melty blood is very loose and it is one of the more enjoyable parts of the game. Knowing your character's moves well enough to hit confirm strange hits and link into combos you've never done before can be thrilling. Having the skill to confirm these hits quickly (especially off non-counter hits) will make you a much better player.
I believe a very good character for learning about the combo system in the game is Sion, if you want to experiment more yourself.