My comments are from personal experience, I used to do engine reconditioning back in the 90s & have fitted many a timesert. When carrying out the work, make sure the piston is at least half way down the bore & no valves are open & in the way of the tap as they are easily damaged (don't ask how I know...).
The kits are horrendously expensive and paying someone who already has the kit would be a much cheaper option.
Should you choose to do it yourself, the kit comes with a stepped tap, the narrow portion is intended to follow the remains of the original thread, it then steps out to the diameter required for the insert, drilling and tapping as it goes. Make sure you keep the flutes packed with grease and keep backing it out, clearing the flutes of swarf & repacking with grease. The insert is driven in using a fluteless tap which expands the bottom of the insert, locking it in position forevermore whilst forming the thread in the bottom of the insert.
I hope this helps and good luck with the repair.
Amy