Key elements in play for impeachment:
Things Trump did as a candidate
Things Trump did during transition
Things Trump did as Presdient
During the election there certainly was interference with Russia in the from of fake news planted in social media.
A lot of it was anti-Hillary like the sex slave in basement of the DC pizza parlor.
There is a theory that Trump campaign officials were working with Russian officials to get any dirt, true or false, on Hillary that could be passed on to Americans through social media. The thought is that Trump either directed this or more likely was aware of it and approved it.
This would be conspiring with a foreign entity to influence an American election.
After the election and before inauguration, Michael Flynn spoke with a Russian official assuring that Trump would ease sanctions put on Russia by Obama after the election.
He has plead guilty to the FBI saying that he lied to them about these discussions and that he was directed to do this by a senior member of the Trump transition team.
If Trump directed this or approved of this it would be a violation of the Logan Act (one president at a time with authority to negotiate with foreign governments).
After inauguration, Trump appointed Flynn National Security Advisor after Obama warned Trump not to hire him because of his Russian ties.
Then Attorney General Sally Yates informed Trump of Flynn's transition discussions with Russia.
Flynn lied to Mike Pence, saying that he had no contact with Russians.
Now we know Flynn also lied to the FBI about this.
And we also know that Trump was aware that Flynn lied to the FBI about this.
Trump subsequently fired Yates and appointed Jeff Sessions who subsequently recused himself from any Russian investigations related to the campaign which he was a surrogate for Trump.
Trump asked FBI director James Comey to back off of the Flynn investigation.
Comey stayed true to his duties and then Trump fired Comey.
As President, Trump is exposed to obstruction of justice.
Trump can be impeached for three offenses:
Conspiracy with foreign entity during the campaign.
Violating the Logan Act during the transition
Obstruction of Justice as President.
The Republican controlled House of Representatives would have to initiate the proceedings.
Then a trial by the Senate would require 67 Senators to vote for conviction which would remove Trump as president and elevate Pence to President assuming Pence isn't tied with those accusations.
Otherwise it could fall to the Speaker of the House, Paul Ryan or whoever the Dems assign of the trial goes into 2019 and the Democrats control the House at that point.
Paul Manafort's efforts during the campaign and subsequent indictment and whatever he says to make a deal could prove the conspiracy or "collusion" side.
But the acts to try and cover that up even if that is not proven could be enough for the other two charges to lead to impeachment.
If Democrats win the House majority in next year's elections there will definitely be impeachment proceedings.
I don't know if the senate would convict but Republicans may want to impeach before Democrats get control so they could assign the Speaker as president if necessary.