Use skill checks
It might help you to think about the steps involved with achieving the end result and how the relevant characters/creatures would react. In your situation, the end result is something like a PC standing triumphantly with one foot resting atop a hogtied zombie, Captain Morgan style.
Obviously, the zombie (to the extent that a mindless undead has any sort of thought process) doesn't want this and so it will resist the PC's efforts. The steps in the process would fall under the rules for contests or contests in combat and might look something like this:
How to hogtie a zombie
1. Get the zombie on the ground.
The basic rules cover grappling and shoving a creature. The PC could attempt to shove the zombie down using an opposed check. This would involve the player using their action to start an opposed check between the character's athletics and the zombie's acrobatics or athletics.
...Except that Zombies, surprise surprise, don't have any skills. This means that they just use their Str or Dex modifier for the check. The reason for this is that skills are just specific aspects of an ability score. That's why the rulebook always reads "Strength (Athletics) check". If the creature doesn't have the specific supplemental skill, it just falls back to their general ability modifier.
If the zombie is knocked to the ground by the shove, move on to step two. If not, the hiding character has revealed her presence and resolves the rest of their turn.
2. Hold it still
Once the zombie is on the ground, that doesn't mean it's going to stay there, content to be tied up. So then the questions become whether or not the party can keep the zombie down, and can they tie the zombie up in such a way that it can't somehow escape?
This is likely a team effort. Somebody needs to keep the zombie pinned while a second character whips out the rope and make an effort to tie rope around the zombie or clap manacles on its limbs, etc.
Once again, you have some opposed checks to roll. I, personally, would have one player need to make a grapple check against the zombie. This works in the same way as a shove would except that the player is attempting to keep a hold of the zombie, not shove it.
Note that, even if the zombie is successfully grappled, that doesn't mean it can't try and bend its head down to bite or kick or scratch in some way. That is, it can still attack even while being held down. In a rules sense, the zombie isn't deprived of its action (ie not incapacitated) by a grapple.
3. Tie 'em up.
Assuming that the zombie has been knocked prone and pinned by a character, somebody needs to tie it up. This character might need to make a Sleight of Hand check to slip the rope around the pinned zombie.
One thing I like to promote at my table is creative application of skills. For example, a player might suggest to me that, since we're talking about hogtying a zombie, might the animal handling skill apply? In situations like this, I like to let the player explain how a skill crosses over into a unique application. The point is to empower the players and allow for some fun storytelling opportunities.
But, of course, the zombie isn't going to make things easy for the player. One option would be to roll some sort of opposed check for the zombie to prevent the bonds from being applied well enough to actually restrain the zombie.
Another might be to just refer to the table of DCs for typical difficulty classes and pick out a value that you think adequately captures the general difficulty of the task. Tying a rope around a zombie that is kicking and flailing might be a bit tricky so I would likely peg that somewhere in the DC 14-16 range (whereas trying to tie up an unconscious prisoner might be a few points easier).
Remember, skill checks are the way D&D determines how successful the players are at doing things. The more complicated the activity, the more checks might be involved. That, in and of itself, is one form of setting the difficulty simply because more checks means more chances of failing to accomplish one of the component steps of the larger task.
One thing that I've really found helpful is to talk through the process with the players and explain your thought process so that they know why they're having to make more than just one check. I've found that players will often times point out that I've overlooked something ("Can two of us hold the zombie down to make it easier to tie up?").
Talking through the checks also gives them hard data that they can use to gauge how challenging a task will be (and whether they want to attempt it or try something else).
For example, they may not know how strong a zombie is, but they do know how strong their character is and how likely they are to succeed in a strength-based check. You can help them understand what they're dealing with even further by giving subtle, narrative clues as to the zombie's strength: "You think that, whoever this zombie was before it became undead was a fairly strapping fellow based on the size of the muscles that haven't yet rotted away."